Photographic Emulsion Technique

PHOTOGRAPHIC

EMULSION TECHNIQUE

By.T. THOR E BAKER, F.R.P.S.,

A.M.I.E.E., F. I ST. P.

1941

J\MERI AN PHOTOGRAPHIC PUBLISHI G CO.BOSTON

ocr1941

CO I'YRIGIIT, 1941, BY

AMERICA PHOTOGRAPHIC PUBLISHING CO.

Made and pn:nted in the United States of America

by the Plinipton. Press, Nortoood, M assachuseus

TABLE OF CONTEJTSINTRODUCTION

IX

1. 1 ATURE OF PHOTOGRAPHIC EMULSIO "SIntroductory - Light-sensitive Silver Com-pounds - The ature of an Emul ion - TheFunction of Ammonia - Sensitizing Propertiesof Gelatin - Ostwald Ripening - The CrystalStructure of Silver Bromide - Iodide in Emul-sions - Grain Size and Emulsion Characteris-tics - The Effect of Light on Silver Bromide

l I. MATERIALS FOR EMULSION MAKING 21Gelatin - Chemicals and Their Choice -Analyses and Tests - Storage - Methods ofHandling Bulk Materials

11 T . LABORATORY EQUIPMENT 37Layout for Experimental Work - CommercialProduction and Its Requirements - Ventilation_ Safelights - Digesting Apparatus - Ther-mostatic Control- Washing and Filtering ofEmulsions - Making up - Cold Storage

IV. NEGATIVE EMULSIONS 63Their Structure and Composition - Types ofFormula - Preparation of the Reacting Solu-tions - Emul ification - Ripening - Setting-Washing - Final Digestion or Finishing-Making up - Anti-halation Methods - Re-versal Emulsions

V. LO"" EMULSIO "SSensitive Material for Copying, CommercialWork and Transparencies - Methods of Pro-ducing High Resolution and Fine Grain-

86

vi TABLE OF CO TENTS

Chloro-bromide Emulsions - Chloride Emul-sions - Mixed-jet Emulsification - Develop-ment of Warm Tones

VI. COLOR-SENSITIVEEMULSIONS . . . I03

Action of Color-sensitizers - Orthochromaticand Panchromatic Plates and Films - Self-fil-tering Emulsions - Hyper-sensitizers - Bi-packs and Tripacks - Three-layer Color Films- Dye-couplers and Color Formers

VII. X-RAY AND ULTRAVIOLET. 128Xvray Emulsions - Intensifying Screens-Lippmann Emulsions - Ultraviolet Plates

VIII. COATINGEMULSIONS0 T GLASS. . . 138Preparation and Cleaning of the Glass - Sub-stratums - Drying Cupboards and DryingProblems - Coating Heads - Ventilation andHeating

IX. BROMIDEANDCHLORIDEPAPERS 155Nature of the Raw Paper - Paper Tests-Baryta Coating - Emulsion Formulas - Labo-ratory Methods of Coating and Drying - Com-mercial Coating Machines - Drying Tunnels-Non-stress Coating - Drying, Reeling andPacking - Variable Contrast Papers

X. FILMS, EGATIVEANDPOSITIVE . .. 175Types of Base - Substratums - Substratum-ing Machines - Negative Film Emulsions-Positive Film Emulsions - Experimental FilmCoatings on Roll and Cut Film - Film CoatingPlant - the Drying Tunnel- Air-condition-ing - Films for Imbibition Emulsions - Static- Commercial Defects - Packing

XI. PRINTING-OUTEMULSIONS. . . 192Salted Paper - Sen itizing Silk - Printing-out

TABLE OF CONTENTS vu

Emulsions - Gelatino-chloride Papers - Collo-dio-chloride Papers - Self-toning Papers - Sil-ver Phosphate Papers

•• l l , TESTING EMULSIONEDPRODUCTS 202Testing Equipment - Estimation of Speed andQuality - Interpretation of the CharacteristicCurve - Measurement of Color-sensitivity-Photometers and Density Meters - KeepingTests

.• l l T. VARIOUSMETALLICPROCESSES. 236Carbon and Carbro Tissue - Gum-bichromate- Iron Printing Processes - Ferroprussiate-Cyanotype - Kallitype - Platinotype and Pal-ladiotype - Diazotype Papers - Bleach-outColor Processes

. "V. EXTREME-SPEEDEMULSIONS 253Super- and Hyper-sensitizers - Sulphur Com-pounds 111 Gelatin - Anti-fogging Agents-Speed Characteristics

I DEX 259

I TRaDUCTION

THE pioneer days of photography were noteworthy for thefact that the sensitive material which recorded the camera

image was in a state of continual evolution. The dramaticchange from collodion to gelatin emulsions due to Dr. Maddoxin 1871, introduced a new era in the art of photography becauseof the mysterious increase in speed which the use of gelatinbrought about. Although so many years elapsed before the na-ture of the gelatin sensitization was discovered, empirical workwith gelatin emulsions brought about a constantly increasingsensitiveness to light, which culminated in the early part of thiscentury in a speed of about 400 Hand D, or its equivalent of10 Weston. Twenty years later the speed of photographicplates jumped up to half as much again, and alter anothertwenty years modern color-sensitive emulsions with their super-sensitizers made another remarkable jump to something betweenfive and ten times the old figure.

In the meantime, the application of photography to technical,industrial and scientific problems has expanded on a truly amaz-ing scale which needs no emphasis here. Physical chemists,physicists and mathematicians have been attracted by the fas-cinating nature of the phenomena associated with sensitiveemulsions into a new field, with which this book attempts insome part to deal.

During an experience of more than thirty-five years in theemulsion-making field, the author has been approached on nu-merous occasions with requests for sources of information onphotographic emulsions. In many branches of experimentalwork and scientific research, it is felt that it would be of un-

x I JTRODUCTION

doubted benefit and convenience if serviceable emulsions couldbe made on a laboratory scale, and suitably coated. As timeproceeds, many newcomers enter the field of commercial pro-duction of one kind or another, and in the following pages theproblems of the investigator and the manufacturer are fullydiscussed. The investigator is obliged to buy in the open mar-ket the available material which happens to possess character-istics nearest to his requirements. The factory laboratory, onthe other hand, makes a considered choice of materials andchecks up on that choice with manufacturing scale tests. Apartfrom actual manufacture, the making of emulsions provides ascope for experiment pregnant with possibilities. Some knowl-edge of emulsion chemistry is of the greatest value in the general'study of photographic processes.

In the pioneer days, photographers had to make their ownemulsions, and it was by the combined efforts of a small coterieof experimenters at the end of last century that the modern (( dryplate" came into being. Some of these men were pure ama-teurs, others were practical chemists and men of scientific train-ing. Out of this little band of experimenters came the foundersof most of the original commercial emulsion-coating factories.The inevitable then happened. The businesses came into com-petition with one another, and further advances gradually be-came guarded as trade secrets. There came a spectacular lullin the hitherto prolific literature, and the textbooks of Dr.J. M. Eder and Sir William (then Captain) W. de WivesleighAbney were the chief fodder on which the would-be experi-menter had to feed.

The interest in emulsion chemistry was renewed in I920 andonwards. The classic work of The Svedberg and other dis-tinguished physicists introduced new methods of attack on themorphology of the silver halide grain. A little later, at a ParisMeeting of the International Congress of Photography, Dr. S. E.

INTRODUCTION Xl

Sheppard announced his discovery of the effect of the presencein gelatin of allylisothiocyanate - the secret of speed had beenrevealed. But the millennium had not arrived, for speed couldnot be increased ad libitum by just increasing the proportion ofsensitizer used in the preparation of the emulsion.

The sensitiveness to light of the emulsion depends to quite animportant extent on the particular gelatin used, and its inher-ent (( impurities." But it is also dependent on the method ofprecipitation of the silver halides, on the method and degreeof ripening, and on the digestion which the emulsion receivesafter the by-products and excess solvents of silver bromide havebeen removed by washing. Modern technique, however, makesit possible to control within fairly definite limits the characterof an emulsion, so that by predetermined conditions the manu-facturer can produce a plate or film having a long scale of grada-lion for the pictorial photographer, a process film suitable forline work, an emulsion specially responsive to X-rays, a soft-working bromide paper for enlargements, or a contrasty chloridepaper for the photo-finisher. Add to this the power whichVogel's discovery of color-sensitizing gave to the industry, andwe find a further field for experiment of vast potentialities,which has helped of course in large measure to provide thevarious solutions to the problem of natural-color photography,and to improve the graphic arts.

While the amateur emulsion-maker cannot compete with themanufacturer of sensitive materials, the latter cannot be ex-pected to interrupt works routine by supplying small coatingsof experimental emulsions made to some specification outsidehis own range of products. This book is thus intended not onlyto be a guide to practical emulsion making, but as a textbookfor technical students, industrial chemists and photographersgenerally, who are anxious for their own reasons to prepareemulsions of various types and speed. With careful work, ex-

Xll INTRODUCTIO J

cellent results of repeatable quality can be made on a smalllaboratory scale, and the apparatus involved need be neitherelaborate nor costly.

The great majority of formulas given in the f@llowing pageshave been quoted from published communications, but it hasbeen the special aim of the author to set out the fundamentallines upon which such formulas are based, so that the emulsion-maker can construct his own for any desired purpose. Newneeds for special characteristics crop up with great frequency,and there is definite scope for the manufacturer of sensitizedproducts who is able and willing to meet these new demands,which may not necessarily run into immen e production figures.

The manufacturer is helped today very largely by modern air-conditioning equipment, improved means for temperature con-trol, excellent emulsion-making plant and coating machinery,and above all by the splendid researches in emulsion chemistrywhich have been carried out in recent years. With the furtherhelp of modern sensitometric work, special packing materials,checking by trained technicians, and above all the ever-growingintelligence of the photographer himself, the sensitive materialsindustry has reached a high state of efficiency, and its productshave assumed the character of precision instruments.

Grateful acknowledgments are given to the Research Labora-tories of Eastman Kodak Company, and Kodak, Ltd., for photo-graphs kindly supplied; also to Messrs. T. H. Dixon and Co.,Ltd., and Messrs. W. Watson and Sons, Ltd., for illustrations ofplant and apparatus; to Burt H. Carroll and Donald Hubbard,whose research papers have been frequently quoted; and to Mr.Charles A. Silver for his assistance with the proofreading.

CHAPTER INATURE OF PHOTOGRAPHIC EMULSIO Slntroductory _ Light-sensitive Silver Compounds - The Nature of anEmulsion - The Function of Ammonia - Sensitizing Properties of Gela-tin _ Ostwald Ripening - The Crystal Structure of Silver Bromide-Iodide in Emulsions - Grain Size and Emulsion Characteristics - TheEffect of Light on Silver Bromide

THE image formed by the lens of a camera is recorded ona light-responsive surface which in present-clay photo.g-

raphy takes the form of sensitive silver salts in a film of gelatm.TO visible action takes place in this film during a normal ex-

posure. But it is capable of providing ultimately a ~isible a~dp rmanent image, obtained in practice by developJl1~ the 111-

visible or latent image formed by the action of the light-raysin exposure. Daguerre showed that by exposing in the cameraa plate made of polished silver treated with iodine vapor, ~ninvisible latent image was formed which could be rendered VIS-

ible by development with mercury vapor. The mercury de-posits itself on the parts of the surface exposed to light, butdoes not adhere to the unexposed parts. The first reference tophotographic sensitometry was probably made by Arago in hisstatement that" to the portions which represent the halftonesthe mercury affixes itself in greater orless quantity proportionalto the action of the light upon these parts."

The thin film of silver iodide formed by the action of iodinevapor on silver, according to the equation

A" + I = AgI,

was in no sense an emulsion, but merely a stain similar to thediscoloration of a brightly polished silver article exposed tothe sulphurous atmosphere of a city. The approach was nev-

2 PHOTOGRAPHIC EMULSIO TECHNIQUE

ertheless being made towards the emulsion, but it was yearslater when Niepce de St. Victor coated glass plates with a mix-ture of albumen and starch ,in which were suspended the sensi-tive salts. Three years later came the wet-cello ion process.In this, a solution of guncotton dissolved in ether and alcoholand containing soluble bromide and iodide was poured over aglass plate and sensitized by flowing a solution of silver nitrateover the set, but still damp, collodion film. The plate was ex-posed immediately in the camera, so that the soluble excess saltsshould not crystallize out.

In an effort to cut out the sensitizing process (still largelyused today') , the first actual emulsion was produced. This wasa suspension of silver bromide or iodide in collodion, made byBolton and Sayce. Then came the use of gelatin by Dr. Mad-dox, and today we find that the great bulk of sensitive photo-graphic materials consist of gelatin" emulsions," or uspensionsof silver salts in a solution of the protective colloid gelatin, ap-plied to glass, celluloid or cellulose acetate film, paper, and morerecently to thin aluminium alloy sheets.

It will thus be gathered that a modern photographic emulsionconsists of a suspension of silver halides in gelatin, which isapplied to a suitable support. It should be noted that emulsionshave been made by Staud and Connelly by dispersing the silverhalide in a mixture of gelatin and a water-soluble cellulose estersuch as cellulose acetate or lactate," while Sheppard and Houckhave used the potassium salt of cellulose acetate diphthalate."Agar-agar has been employed with little success. In spite ofwide research, no compound or substance has yet been dis-covered which is anything like as sensitive to light as bromideof silver, yet extreme sensitiveness can be obtained only whenit is subjected to the influence of gelatin, and if chemical fogis to be avoided on development, a small proportion of theiodide of. silver must accompany the bromide.

ATURE OF PHOTOGRAPHIC EMULSIO S' 3

The salts are formed by double decomposition, as for ex-ample:

AgNOa + KBr AgBr + K °3silver + potassiumyields

silver + potassiumnitrate bromide bromide nitrate

While from the chemical equivalents it would seem that I70

parts by weight of silver nitrate would react with I I9 parts ofpotassium bromide, it is necessary in practice to have presentan excess of soluble bromide, for reasons which will be discussedlater. If an excess of silver nitrate were present, then the emul-sion would print out on sufficient exposure to daylight, givinga visible image of metallic silver; for this purpose, however, thechloride of silver in combination with an organic silver sa.lt iscommonly used. Silver chloride without excess of silver nitrategives, on the other hand, a comparatively insen itive emulsionof the type used in making chloride or " gaslight" papers andslow transparency plates, which yield on alkaline developmentblack and white images having great contrast and brilliance.

Thus the three halides, silver bromide, chloride, and iodide,uspended in gelatin or other suitable colloid medium, provide

us with the light-sensitive emulsions of which this book willlargely treat. Trivelli and Sheppard 3 describe a series of mer-curic iodide emulsions, their sensitiveness and density-givingpower being" much inferior to silver bromide." For printingprocesses on paper, certain salts of iron, copper, thallium, etc.,can be used, as also can certain diazo compounds. The out-tanding feature of a silver bromide emulsion containing iodide,

however, is the .fact that by the use of ammonia and the ap-plication of heat under controlled conditions, a remarkably ex- _tensive range of speed, density-giving power, gradation, and.ontrast can be obtained, which is repeatable with considerableexactness.

Reduced to its simplest form, the making of a sensitive emul-


sion has a good deal of resemblance to cooking. Solutions ofthe reacting salts are weighed out and mixed, one of them con-taining a little gelatin which acts as a protective colloid andas a means of preventing sedimentation of t~ halide precipitate.Any amateur cook knows the difficulty of making 'a goodmayonnaise or hollandaise sauce. Some of the tricks of theemulsion-maker are comparable to those of the adept culinaryartist, and the sensitometric characteristics of the finished emul-sion largely depend on the exact method of mixing and cooking.The mixed emulsion is kept warm for a certain time, duringwhich it is said to " ripen," more gelatin is then dissolved in it,ana it 'is cooled until gelled.

Reference to the equation of the chemical reaction givenabove will show that soluble nitrate (KNOa in this case) isformed along with the silver bromide. Any such nitrates would.crystallize out on drying, if allowed to remain in the emulsion'when coated on glass or film base, and such by-products mustbe removed. To effect this', the emulsion is broken up in thejelly stage into small pieces, known as shreds, worms, or noodles,'and these are su pended in water until the soluble by-products,together with any excess of ammonia and bromide used in themaking, are washed out by diffusion. The washed gel is thendissolved by heat, fresh gelatin is usually added, and the emul-sion is again cooked, this time at a higher temperature which'is more or less critical, as is also the precise time of the cooking,digestion, or " finishing."

It is the variations in the proportions of ingredients, the ex-act method of precipitation, the time allowed for ripening, thetime of cooking after washing and the temperature employed,and the character of the gelatin, which influence the final char-acteristics and give us the immense range of modern emulsions.These vary from the very slow silver chloride paper emulsionsused for handling in Mazda light, to the exceedingly fast films

I'

\.

I

.I

,NATURE OF PHOTOGRAPHIC EMULSIO S 5

~hat record images in the millionth part of a second. A roughIdea of the relative sen itiveness of various types of emulsionsis given by Clerc " as follows-

.

Ultra-rapid nezative emulsions " 75,000 to roo,ooo *Positive emulsions, black tones. . . . . . . . . .. 1,000 to 3,000

Gelatino-bromide papers, warm tones. .. . . 300 to 1,000

Gelatino-chlorobromide papers 100 to 200

Transparency plates I to 25

Gelatino-chloride papers I to 5

Silver bromide emulsions made with gelatin are very muchhigher in sensitiveness than those made with collodion. Thedigestion of the silver bromide grains or crystals with the gela-tin is obviously the prime cause of this difference. Clerc tellsus 5 that, following from an observation by R. F. Punnett in1924, S. E. Sheppard found that the differences in sen itiveproperties are due to the presence in gelatin in varying propor-tions (from r in 200,000 to r in r,ooo,ooo) of sensiiizers, among~hem.being thiosinamin (allyl thiourea), and mustard oil (allylisothiocyanate ). The presence of these sensitizers, occurringnaturally in gelatins, and in quantities varying with differentmakes or different batches of anyone brand of gelatin, explainsthe great variation in quality of any given emulsion made toone formula with different samples of gelatin - a behaviorwhich greatly perplexed the early emulsion makers and for along time made impossible any assurance of uniformity in their~roducts. ~xperiments made with gelatins rendered chemicallym~rt, to which have been added known sensitizers, such as allylthiocarbamide, sodium thiosulphate, etc., have given resultsqualitatively very similar to those made with untreated" ac-tive" gelatin, though the characteristics of active gelatin cannot be explained by allyl thiocarbamide alone." A great dealof research has been conducted in recent years on the chemistry

* This figure must be increased today as the result of recent advances.

j

6 PHOTOGRAPHIC EMULSIO TECH IQUE

of these sensitizers, and, any real advance on present speeds willbe due to new discoveries in this field rather than to modifica-tions in ripening, digestion, etc. Elaborate efforts have beenmade, also, to produce some standard type of gelatin, withlittle success. As gelatin is an amphoteric colloid, capable ofcombining with either anions or cations depending on the hy-drogen ion concentration of the solvent medium, a good deal ofwork has been done, by Sheppard and others, with emulsionsmade with iso-electric gelatin, the pH of which is 4,7, Thetreatment to which gelatins are subjected by certain manufac-turers is kept secret, but as a general practice the choice of abatch from a number of samples by actual experiment is theaccepted guide of its suitability for a particular purpose,

As an example of the versatile nature of emulsion grain sen-" sitizing, the suggestion may be quoted of a mononuclear or poly-

nuclear heterocyclic compound containing a hydrogen atomlinked to nitrogen but replaceable by silver. The pyrimidenes,pyrazols, and purpurines are quoted as examples.

The great majority of speedy emulsions are made with am-monia, which assists in the ripening or crystal growth of theprecipitated grains of silver halide and their subsequent sen-sitivity. In another type of emulsion, however, the suspensionin gelatin is " boiled," which means actually that it is heatedat a temperature somewhere between 160° F .. (70° C.) and afew degrees below boiling point. Such an emulsion is an inter-esting one to consider at this point, because the absence of am-monia enables us to watch the process with less chemical com-plications. Trivelli and mith 7 outline such a formula. Twosolutions are prepared as follows:Potassium bromide. . .. . . 165 gPotassium iodide .".... 5 gGelatin ... , .. ,'.,...... 65 gWater """""""'" 1700 cc

Temperature, 700 C.

Silver nitrate, ' . , , ... , " 200 gWater """""""'" 2000 cc

Temperature, 720 C.

NAT RE OF PHOTOGRAPHIC EMULSIO S 7

In a series of mixings, the silver nitrate 'solution was added bypouring it through different sized nozzles into the salts solution,t he times of precipitation varying from 3 I seconds to 85 min-utes, 10 seconds. The emulsion was ripened for twenty minutesat 70° C., then cooled quickly to 45° c., when 250 grams ofgelatin previously washed in water was added and the wholestirred for twenty minutes at 45° C. After standing overnightin a cold storage room, it was washed and remelted to 42 ° C. andmade up to a weight of 6.3 kilograms by the addition of IOO

grams of fresh gelatin soaked in the requisite amount of water.The final pH was 6.54.

The relation between the time of precipitation and the aver-age grain size is interesting, and is shown in the table below:

Time of precipi- I Total surface ofEII/ulsion No, of grainsnumber talion. X 10-9 per cm3 grains 'in cm.' per

Min. Sec, cubic centimeter

8 0 31 6,85 3°539 4 22 2,09 1957

TO 10 12 0,63 102611 19 30 0,28 75612 42 40 0,17 564T3 54 IS 0,09 66314 85 10 0.04 320

I'igure I shows the relation between grain size and time takenror precipitation, reproduced from the paper quoted. It wasfound by Southworth 8 that there is a quantitative relation be-(ween the Hand D speed (p. 208) and 'Y (maximum contrastOil complete development), and this was found by Trivelli andSmith to hold good for a number of emulsion series with dif-Ir-rent times of development.

l.ct us now examine the making of the emulsion after theIllit ial precipitation of the silver bromide, usually termed ther-mulsiflcation. As already stated, a great proportion of speedy

8 PHOTOGRAPHIC EMULSION TECHNIQ E0

5 --0 »>

.-" ~.___1-'" '"5 .... ~

~I--"""

TIME OF PRECIPITATiON

FIG. I

emulsions are made with ammonia. In many cases the wholeof the silver nitrate in the formula is treated with sufficientconcentrated ammonia to \redissolve the precipitate of silverhydroxide first formed. The temperature of mixing is then

. u\sually much lower. The AgBr precipitate formed in emul-sification appears as minute shapeless grains under the micro-scope, but as ripening proceeds they grow in size and a definitecrystalline shape is recognized. The crystals continue to in-crease in size, as time proceeds, owing to the presence of thesilver bromide solvents - ammonia and the excess of solublehalide.

Silver bromide, according to von Weimarn's theory, wouldgive a precipitate on mixing the reacting substances, the cc grainsize)) of which would be controlled by the concentration of thereactants. The fact that the precipitation takes place in thepresence of a colloid, however, must be taken into considera-tion, in addition to the excess of AgEr solvents. To obtainuniformity of grain, the emulsion is stirred during precipitation,this tending to reduce the size of the crystals. In some casesof ripening, the crystals are allowed to grow without agitation,especially in those cases where a mixed selection of grain sizesis needed to give photographic latitude, or a long scale ofgradation.

1'1(:. c. GRAINS OF A COMMERCIAL PHOTOGRAPHIC EMULSION X 5,000

--, C;: .•. II ')-- .~:

\" i ~&('.I.'

"r o'a,,~'1CJo,.'.. ,..

10 PHOTOGRAPHIC EMULSION TECH IQUE

In the early stages after emulsification the system tends toreduce its surface energy through any colloidal silver bromidebecoming transformed into crystalling AgBr. By the so-calledOstwald ripening, the larger crystals grow at the expense ofthe smaller ones, and the longer the ripening proceeds, thebigger do the crystals or grains become. A certain amount ofaggregation eventually takes place, unless stirring is given. Adrop of emulsion can be taken on a glass rod, smeared over aclean glass slide, and thinned out with a few drops of hot water.The slide is then sharply shaken free of excess fluid, leavingonly a single layer of grains adhering to the surface. The speci-men, as soon as dry, can be examined with a twelfth-inch oil-immersion objective, using the concave mirror and small C011-

denser aperture. It will usually be observed that a wide varietyof, grain sizes is present, appearing as hexagonal plates, tri-angles, needles and unresolvable particles (see Figs. 2 and 3).All of these are actually octahedra. Dr. Sheppard states thatin 122 different emulsions examined at a magnification of 2,500

diameters, only octahedra could be positively identified. Theequilateral hexagons which so soon become familiar to anyoneexamining emulsion grains with the microscope are plates de-veloped in two directions, while needles appear which are de-veloped principally in one direction - all, however, octahedra.

If a few drops of the ripening emulsion be smeared over astrip of glass and looked at by transmitted Mazda light, thecolor after emulsification will be orange. As ripening proceeds,this turns to yellow, then greenish-blue and then to blue, atwhich stage the maximum useful ripening will usually havebeen reached. If ripening be allowed to proceed further, the'transmitted light may appear bluish-violet or gray; at this stagethe emulsion will have overgone the mark and will probablyshow fog on development without having gained any advantagein speed. This method of watching the progress of ripening

1111:.3. GRAINS OJ; A PURE BROMIDE PHOTOGRAPHIC EMULSIO X 5,000

I2 PHOTOGRAPHIC EMULSION TECHN IQUE

was much favored by the old school of emulsion chemists, atrained eye being able to detect with considerable accuracywhen the optimum stage had been reached. As the time canvary from twenty minutes or so to three or four hours withan unfortunate change in gelatins, it is a useful check in thefactory when new batches come into use, even though they mayhave been passed as O.K. in the laboratory.

So far we have said little about the iodide in an emulsion.Reference to any formula for a fast emulsion will reveal thata small amount of silver iodide is used. Whether this is formedat the time of emulsification or at a later stage, the iodide ap-pears to take a definite part in the molecular structure of thebromide. Iodine has a greater _chemical affinity than brominefor silver, and at whatever stage it is introduced, it will replaceit. The' term iodo-bromide is frequently used. X-ray crystalmeasurements of the lattice spacings indicate a complete homo-geneity throughout the crystal, but the presence of the iodideions changes the spacing, making the distance between themlarger than in the pure state. This would indicate, according toFriedman," that a crystal which contains iodide as well asbromide ions exists in a strained condition, the degree of strainbeing dependent on the concentration of the iodide ions withinthe crystal. To a slight extent this corresponds to the sen-sitivity of the resultant grain, for it has been determined thatthe maximum sensitivity is attained when the concentration ofthe iodide is approximately four mol per cent of the total halidecontent. Beyond this concentration the sensitivity falls offagain.

The range of diameters of the crystals tells a good deal aboutthe character of the emulsion that is to be. A wide range in-dicates good gradation and latitude, while if the crystals areall of similar size, a high gamma with early reversal and lackof latitude ordinarily results, except in cases of very slow emul-

TATURE OF PHOTOGRAPHIC EMULSIONS I3xinn which have had special treatment. While the graingrowth in ripening is almost invariably accompanied by an in-e rea e in speed of the finished emulsion, the final sensitivityis determined more by the conditions under which emulsifica-Iion took place than by the ripening itself, provided always thatIh final cooking or finishing is carried to completion. Theearly literature on emulsion making was concerned chiefly withIhe endeavor to get all the peed before washing. It is nowan open secret, since the paper of Carroll and Hubbard waspublished in the Bureau of Standards Journal o] Research in193 I, that a great deal of the final sensitivity is obtained incommercial practice by the cooking given to the washed emul-sion. Trivelli and Smith disclose this fact still further in Corn-munication No. 704 from the Eastman Kodak Laboratories.

The nature of the gelatin undoubtedly plays a very impor-Iant part, as well as do the concentration of the reacting solu-Iions and the method of pouring and stirring. As an exampleor the extraordinary effectiveness of emulsifying condition, onemay take a formula, built on lines originally published by J. M.Ecler, where the silver nitrate is merely wet with water, red is-solved with ammonia, and in this very concentrated conditionis literally" flopped" wholesale on to the salts solution in thejar. This, of course, gives an instantaneous precipitation, thegrains being of a very uniform size. 0 ripening time what-ever is allowed, the bulk gelatin being added immediately andthe emulsion set in ice water as soon as it is dissolved. Suchan emulsion yields a film of very high gamma but of poor lati-tude, specially suited to copying black and white work. It willIhu be gathered that the emulsion chemist must find out forany emulsion formula the optimum time of ripening and the nee-cssary physical conditions to obtain the results he wants, asalso the exact m~thod of emulsification, in addition to selectingthe most suitable brands of gelatin.

PHOTOGRAPHIC EMULSIO TECH JIQUE

Let us assume now that an emulsion has been ripened and thatsufficient extra gelatin has been added to give it the necessaryviscosity for coating - say a total of eight or nine per cent ofthe volume. This viscosity is of course arranged to suit thetype of plate, film or paper, and the particular coating ma-chine and climatic conditions. The soluble by-products to-gether with any excess of ammonia and the excess alkali halides ,are next removed by washinz, While this is ordinarily done inthe manner described in detail in Chapter IH, other methodshave been suggested, such as pouring the emulsion in a finestream into a large bulk of alcohol, thereby coagulating or pre-cipitating the gelalino-bromide, the water with its soluble saltsbeing taken up by the spirit. Many variations of the spiritwashing method were described in the early literature, espe-cially in the British Journal of Photography, towards the end ofthe last century.

The washed emulsion, then, is a dispersion of silver halidegrains in more or less pure .gelatin. The gelatin neverthelesscontains sufficient sensitizers - or they have been already ad-sorbed to the silver bromide crystals - to insure that when theemulsion is remelted and heated the maximum sensitivity isobtained. That some sensitization has been effected within thegrain in the case of mixed-grained emulsions has been shown bythe fact that while after exposure the smaller grains can berendered ·undevelopable (the latent image destroyed) by treat-ment with chromic acid, some of the larger grains still remaindevelopable. The final cooking, when the speed of the washedemulsion can be increased many hundreds or even thousands oftimes in the course of thirty minutes or so, is known as digestionor finishing, and is fully dealt with in Chapter IV.

Owing to the lack of emulsion-making literature, particularinterest attaches to the paper by Trivelli and Smith 10 in whichare described experiments to determine the influence of grain

ATURE OF PHOTOGRAPHIC EMULSIO S ISsize on the final digestion. These experiments indicated twothings; (a) the Hand D peed of the smaller-grain emulsionsincreased to a greater extent than that of the large-grain emul-sion ; (b) the gamma of the smaller-grain emulsions increasedless than that of the larger-grain emulsions. As an exampleof (a), the figures below are quoted from the paper of theseauthors:

Average grain size in jJ.2 0.r6 I 0·54 I 2·93

Time of finishing at Increase in speed600 C. in minutes

ro 3-4 3.8 0.820 4·6 5.0 0·930 5.2 4·4 0·9,

5·9 4·7 1.040

Average grain ize in jJ.2 0.r6 I 0·54 I 2·93

Time of finishing at Increase ·in gamma600 C. i It minutes

10 J.4 J.6 2·520 1.$ 1.7 2·9-, 30 1.5 1.9 3.240 l.S 1.9 3.2

'\..vdrnitting that gupn ~lze ~leed not be the d.eCldll1gfact~r ~n

Ihe peed of an eni ilsion, It has been established that within.-xperimental limits here is a direct proportionality betweent11(' average grain siz and the Hand D speed in certain emul-ion series. On the ther hand, as the gamma of very fine-

grained emulsions does ot increase as much on final digestion asil does in the case of coa 'ser-grained ones, it is usual practice toccure the high contrast here desired) of a slow fine-grained

r-mul ion in the mixing and ipening rather than to trust to thed ig('stion after washing. Seed being due largely to the sen-

16 PHOTOGRAPHIC EMULSIOl TECHNIQUE

si.tizing of the silver halide by the gelatin, it is obviously moredlf~cult for the gelatin to function in the case of fine grains,which have a very large surface as compared to the coarsegrains. As an 'example in the increase in surface rea of a arainwith decrease in diameter, the illustration may be given "'of acube of 1 cm side, which has a free surface of 6 sq. cm. If di-vided up into a corresponding number of small cubes each of0.01 thousandth of a millirneter side, the free surface of the6 sq. cm would be expanded to an aggregate of 60,000,000sq. cm.

Reference may be made here to the effect of very smallquantities of sodium sulphite in emulsions. If this substancebe introduced in quantity sufficient to reduce a few tenths ofone per cent of the silver bromide present, and the emulsion?e t~en digested, it can act as a powerful sensitizer, approach-1l1? 111 large measure the effect of the natural sensitizers in gel-atl? As stated by Carroll and Hubbard.v- the sensitivity nu-clei formed are of metallic silver, in amounts similar to thatof the silver sulphide nuclei of normal emulsions. The rate ofchange of sensitivity increases with increasing alkalinity, anddecreases with increasing bromide-ion concentration. The rateof after-ripening with sulphite is less affected by hydrogen ionconcentration than the corresponding process in active gelatin.The authors state that while many of their' emulsions sensi-tized with sulphite were of " commercial quality," none wereequal to the best obtainable by standard methods from thesame type formulas.

After the emulsion has been coated on its support, a certainamount of after-ripening or ageing can go on, especially if theripening after emulsification has not been carried to, or nearto, an optimum point, or if the finishing was not carried to fi-nality. This gradual rise in speed or gamma in a coated plateon keeping is not to be confounded with the after-ripening

NATURE OF PHOTOGRAPHIC EMULSIONS 17

which Carroll and Hubbard define as " the increase in sensi-1ivity of photographic emulsions after washing," which we haveseen depends on the effect of heat treatment upon the initiallyripened grains.

The ageing of plates and films may be regarded as a con-Iinuation of digestion in the dry state and at storage tempera-Iure, but for our purpose it should be regarded more as deterio-ration. A coated product, after keeping for a short time to getinto complete equilibrium and for the hardening agent to com-plete its work, should maintain the state in which it is put outfor two to three years, and much longer in the case of slowmaterials. On the other hand, there may be a tendency in thecase of films for the gradual lowering of pH, due to acid libera-Iion, when speed will be lost. This is a matter to which due.utention must be paid in commercial film manufacture. As:\11 instance of the excellent keeping of glass plates, the casemay be quoted of star drift measurements at Oxford Observa-tory, England, where special rapid plates were exposed in anustronomical camera on a group of stars, and without develop-111 ntwere stored for \:I.~en years, and then given a secondrxposure at the completion or--the orbit. The displacement be-Iween the two images, one havin?be~ latent fo.r fourteen yearsbefore development, was the measure Oi~'he dnft.

When light is absorbed by a grain of si er iodo-bromide, thesilver halide molecule becomes dissociate and atomic silverund free halogen are formed, the halogen being absorbed by1h acceptors in the surrounding gelatin. he new system canhe described as a crystal of silver iodo-br mide upon whichatornic silver is adsorbed. Such a system is sily reduced tometallic silver by the action of the developer, as originallyestablished by Carey Lea. That the reduction tarts from thesensitive nuclei was shown in 1922 by the brilliant photo-micrographic resear hes of The Svedberg.


The metallic silver grains formed from the exposed silverhalide by reduction during development have been regarded astiny coke-like masses, which link up to form aggregates orlarger" grains," but it has been shown by recent work with theelectron microscope that the grain formation is actually ribbon-like in character. Photomicrographic work done in the ordi-nary way up to magnifications of X 2000 and upwards (diam-

FIG. 4

eters) has never revealed, nor indicated, such a formation.But with the immensely greater magnification of X 50,000, pos-sible with the electron microscope, definite results have been se-cured which may necessitate reconsideration of some of ourpresent ideas. The image grain has now been shown (Fig. 4) toconsist of a ribbon of silver crumpled up like a string, whichwould appear to occupy a part only of the total area of what wehave regarded as the grain and to possess light transmittingproperties rather than being completely opaque. It is more thanlikely that subsequent studies along these lines will show thestructure to vary as regards its nonopacity with different condi-

NATURE OF PHOTOGRAPHIC EMULSIONS- 19

tions of emulsification, and thereby be responsible to some ex-tent for characteristic variations.

It is beyond the scope of this book to deal with the subjectof the latent image and the theory of development, but itshould be emphasized that one reason why sensitive silver emul-sions have made modern photography possible and of such im-mense value to scientific research, is that over a long range oflight intensities, depending on the individual emulsion, equalincreases in exposure result in equal increases in opacity of thedeveloped image, or equal increments of " developability."

Equal effective exposures produce equal densities under con-trolled development (Bunsen and Roscoe reciprocity law), ex-cept for a correction indicated by Schwarzschild which makesit necessary to substitute for the exposure I X T (intensitymultiplied by time) I X TJl, P being the Schwarzschild factor.An example is given by Strong," showing to what extent theBunsen and Roscoe law fails in the case of motion picture posi-tive film. For a range of illumination intensities from I to33,000, p varies from 0.68 to 1.00, the maximum intensity beingr3 r lumens per square meter and the exposure time varyingbetween r8.2 hours and 2.5 X 10-4 second.

The covering power and maximum density of an emulsiondepend to some extent upon the size of the reduced silver grains,their aggregation, and their number per unit area, the ratio ofgelatin to silver having some effect where density measure-ments are made by specular light. Clerc states 13 that an opti-cal density of I (transmission ten per cent) on a photographicnegative, corresponds to a mass of silver of about ten milli-grams per square decimeter, a mass which is variable with .thegrain size, conditions of development, and the wavelength ofthe radiations used. Trivelli and Smith showed H that the de~crease in gamma of an emulsion is proportional to the squareroot of the total number of grains. They state that this de-

-.

I1

20 PHOTOGRAPHIC EMULSro TECHNIQUE

crease is considerably greater than the changes observed in theresoluing power by diluting the emulsion. For recording pur-poses, a good black which will give sufficient differentiation inp.rinting or on projection must be combined wit;h high resolu-tion, and the latter must therefore depend largely on suitableemulsion characteristics rather than on thin coating. In fine-grain emulsions, however, Trivelli and Smith find that the re-solving power increases exponentially as the grain size is di-minished arithmetically.

Much of the foregoing has applied to « rapid" emulsions ofsilver iodo-bromide having Hand D speeds ranging from 25to 2000 or more. In materials for printing, lantern-slide emul-sions, and copying, speed is of little consequence. Where extra-'fine grain, high resolution and negligible fog are the chief de-siderata, physical treatment rather than chemical is involvedand in chapters dealing with such emulsions it will be seen thatthe general technique of making is considerably modified.

Chapter Rejerences

I. Staud and Connelly, V.S.P. 2, 127, 621, 1936.2. Sheppard and Houck, V.S.P. 2,.127,573,1936.3· Trivelli and Sheppard, The Silver Grain of Photpgraphic Emulsions.4· L. P. Clerc, Photography, Theory and Practice, edition 1937, p. 358.5· Ldem., p. 132.

6. Carrel! and Hubbard, Bur. of Stands. Tourn; of Research, 7, Aug.,193I.

7· Trivelli and Smith, Communicn. 699, Kodak Research Labs.8. Southworth, Brit. J. Phot., 84, 61I, 1937.9· J. S. Friedman, A-mer. Phot., 34, 4, 292.

10. Trivelli and Smith, Photo Journ., 79, 609, 1939.1I. Carroll and Hubbard, Bur. of Stands. Tourn. of Research, II, Dec.

1933·12. J. Strong, Procedures in Experimental Physics.13· L. P. Clerc, Photography, Theory and Practice, p. 139.14· Trivelli and Smith, lac. cit.

--

CHAPTER II

MATERIALS FOR EMULSIO MAKI GGelatin - Chemicals and Their Choice - Analyses and Tests - Storage- Methods of Handling Bulk Materials

GELATI is the material which presents the most prob-lems to the emulsion chemist. Its physical and chemical

properties can be tested in the laboratory, but the physical char-acteristics are apt to change according to the treatment it under-goes, and the chemical properties from an emulsion-makingstandpoint depend to an important extent on certain constitu-ents present only in extremely minute quantities. Boguestates 1 that the gelatin solution used in an emulsion must beso made that the dried film will have just the right porosity toelectrolytes, for in all stages of development where chemicalsare used it is necessary that they penetrate and impregnate thegelatin layer with considerable ease, but no trace of the pre-cipitated silver must be permitted to escape. Gelatin acts bothas a protective colloid and a vehicle for the silver haloids, andit is in. fact quite astonishing what a mass of silver haloid canbe suspended in a very weak solution of gelatin with compara-tively slight sedimentation.

The best gelatin for emulsion-making purposes is probablythat extracted from calves' hides, but other hides and otherparts of the animal are used, and bones also. Hides having ahizh fat content are not suitable for emulsion gelatin, accord-oing to Huzii," as it causes spots on the plates. If the tempera-ture in extracting the gelatin is too high, the product will havehigh light sensitivity and fog-giving properties; increase in thenumber of extractions has similar effects,

--

22 PHOTOGRAPHIC EMULSION TECHNIQUE

The preparation of the gelatin involves a number of opera-tions. The raw materials are steeped in Iime water, after hav-ing been thoroughly washed free of dirt and blood. The limingprocess dissolves out the albuminous and mucinous constitu-ents, and, as it is a process of alkaline hydrolysi , caustic alkaliis sometimes employed. The alkali is later neutralized and aprocess of digestion or boiling given, followed by concentrationof the liquor, bleaching, washing, etc. The final jelly is cutinto sheets and dried on strings or wire nets, and here a certainamount of bacterial contamination can be contracted. Leafgelatin is sold in the form of thin sheets, of which in the photo-graphic quality 100 to I la go to the pound, It may be cut intothicker sheets which are ground to powder when dry, or turned,into flakes which are sometimes sold as such. The groundgelatin when sieved gives a powder of about SO per inch mesh.Such powder is liable to contain traces of iron. Photographicgelatin, which may be looked upon as glutin and a mixture ofglutiri and chondromucoid, is the purest form made, being su-perior in quality to culinary gelatin.

A good gelatin should give a clear and nearly neutral solu-tion. The moisture content should not be more than twentyper cent and lower in the case of " soft" gelatins. There areroughly three classes of photographic gelatins, hard, mediumand soft, which can be differentiated by their setting and melt-ing points and the quantity of water they will absorb. The ashcontent of a good gelatin should not exceed two per cent. Basesand heavy metals should not be present in more than traces;these may be sodium, calcium, zinc, iron, copper, arsenic, nickel.Phosphates, sulphates, sulphites, chlorides, borates and sul-phur dioxide may also be present.

It is interesting to note that as early as 1847 Niepce em-ployed gelatin as a vehicle for coating silver iodide 'on glass,though this was soon dropped owing to the effect of the acid

1ATERIALS FOR EMULSIO MALO G .

1101111re of the silver bath. Eight years previously, Mungo Pon-1'111had rendered paper sensitive to light with potassium di-r hrurnate, and later Becquerel showed this sensitivity to be"II(' to the size in the paper. It was Dr. R. L. Maddox who,I11I87 I, first employed gelatin as a vehicle for silver bromide,11I<1so prepared the first emulsion. Its use gave an entirelyIII'W m asure of speed, but it at the same time introduced aI'II'at many perplexing phenomena, some of which are still notIully understood.

1)1".S. E. Sheppard quotes in his monograph on the theoryIII photography, " Gelatin," a statement by V. B. Storr 3 in the\,'r OJ1d Annual Report of the Society of Chemical Industry on1111'progress of applied chemistry, as follows: "The physical(lloflt'Ities are a very insufficient guide to the suitability ofI'I'lalin for making photographic emulsions. There are certain, lu-mical differences between different types of gelatin and even111'1wcen different batches of the same type which are more ef-It'l Iivc in determining speed, freedom from fog, and such qual-IIII''{.... It is very possible, if not probable, that they (chem-11 ,d differences) are due to the presence or absence of veryIII:dl quantities of specific substances rather than to varia-

IlOllSin the proportions of the main constituents of the gelatin."l'his tatement tells a good deal of the story of gelatin, and

I' 1'1Y emulsion chemist will add to it that the only satisfactory\ .ry of testing a new sample is to make with it a semi-works-1/1' lot of the particular emulsion for which it is to be used.

II I <l common occurrence, for example, for a sample whichh,1 1)(' n turned down by one maker to be accepted as excellentI,v .uiother. Where results are to be repeated over long periods,III where works-batches of emulsion are being made, it is sound1'1011Iic to use a blend of two or three different lots or even1I1,t!I'SItaking care that only one of these is allowed to run out1I 11111'time, so that when a new batch of gelatin is worked in,

PHOTOGRAPHIC EMULSION TECH IQUE

only a proportion of the total amount used for each making ischanged.

The chemical test of most importance in selecting gelatin isfor silver reduction. A one; or two per cent ,wlution of thesample is made up with distilled water, and to a measured vol-ume is added an equal volume of ten per cent silver nitrate solu-tion, to which has been added just sufficient concentrated am-monia to redissolve the precipitate of silver hydroxide firstformed. The mixture is well stirred and is then left in the darkfor a fixed time, along with a similar control test of a gelatinof known good quality. Any darkening or formation of a pre-cipitate will give a good indication of the quality of the sample.A good gelatin.will contain a relatively small proportion of non-glues. According to Stelling,s the non-glue extracted from gel-atin by alcoholic precipitation of the glue is 3.39 per cent,S. 73per cent from hide glues, and IQ to 16 per cent from bone glues.Arsenic has some effect on speed and fog production, and hasbeen found to occur in eightout of twelve samples," in the pro-portion of 0.0005 to 0.003 per cent. Copper, zinc, tin and leadmay be found by the usual methods. The most important im-purities are probably the sulphur compounds dealt with else-where. The presence of sulphur dioxide is undesirable, espe-cially if the sample is to be used for highly color-sensitiveemulsions or for the making of light filters. Where leaf gelatinis to be used for the latter purpose, it can be washed in severalchanges of water before use, the leaves being kept well sepa-rated, when soluble impurities will be removed.

Viscosity can be measured in a number of ways. A one percent solution may be tested with an Engler or Ostwald visco-simeter, at a temperature of 120° F. Running the solutionthrough a water-jacketed pipette and comparing the time takenwith that of plain water is another and cruder test, but onewhich will give useful practical information. Thermal hys-

MATERIALS FOR EMULSIO J MAKI JG

1l'II'~is or hydrolysis may upset physical tests, for it must be11'IIH'm\) red that previous treatment such as heating up rapidly.uu l cooling, gelling and remelting and so on, may alter ~he be-h.rvior of a gelatin solution. The viscosity of most gelatins can111' very largely controlled in an emulsion by modification of ther utu i-ntration, by the addition of alcohol or acetone, or by the.nhlit ion of chrome alum, formalin or sodium sulphate.

E F"". sti r-r e r

D

F

--- - - - - - - --- A---

BFIG. 5

I 11(' melting and setting points of a sample are of some im-I" u l.uu e. Emulsion gelatins should have a melting point some-lilll' lxtween 70° and 78° F. (22° and 25° C.) and a setting

I'" 111 Ilf 79° to 86° F. (260 to 30° C.). R. Child Bayley's


method of estimating melting point was to cast small discs ofgelatin upon a flat metal surface, which is afterwards placedin a vertical position and heated. At the temperature of themelting point, the discs begin to slide downwards. The ap-paratus takes the form shown in Fig. 5. This is a copper tankhaving a plain front say twelve by eighteen inches in size, witha smaller back about nine by twelve inches, the depth AF beingabout ten inches. The bottom is inclined as shown in the dia-gram, so that when heat is applied to BC none gets to the test-ing surface ABCD. Two or three thermometers are fitted incorks placed in the top ADEF, the tank being filled with waterand gradually warmed with an alcohol lamp or Bunsen burnerafter the discs have been cast. The discs can be made by pour-

FIG. 6

ing the gelatin solution into a paper cylinder or mold set up onthe surface ABCD, which is placed first in the horizontal posi-tion (see Fig. 6). Tests should be made with solutions of someuniform strength. E. J. Wall recommends ten per cent. Thegelatin solution may be introduced into the molds with a pi-pette, each one being cast of the same height, about one-halfinch. The tank should then be left in the cold for eight hoursuntil the gel is thoroughly set and in equilibrium. The tank is

MATERIALS FOR EMULSION MAKING

1111'11set in the upright position, the paper cylinders cut round\\ lilt a sharp penkife and removed, and the tank filled with wa-I1I. small motor stirrer is advisable to keep the whole frontI11rar uniformly heated. As soon as the melting point has been

11',1(hod, the little discs will begin to slide down the face of theI lid', and the average temperature of all of them will give the1111,11ing point desired.

vnother method, used a good deal by the author, is to intro-dill I' the gelatin solution into four little glass tubes one and one-1J1I:11'(rror one and one-half inches long, and about three-I u-enths-Inch bore. These are set and kept cold, at aboutI ' F. for four hours, and are then tied around the bulb of.111nccurately graduated thermometer. The bulb and tubes are1111'11suspended in a beaker of thin petroleum oil of about 0.90

IlI'rirlc gravity, the beaker standing in an outer water bath.\ -unnll mechanical stirrer keeps the oil in motion. The waterI.I(kct is heated, and as the oil approaches the melting point"I Ilu- g latin, the gelatin slips out of the glass tubes and dropsIII IItI' bottom of the beaker. An average reading of the fourIllill's will then give a fairly reliable figure for the meltingI"11111.

l'he setting point, which is on the averaze eight to ten degreesI' .ihu-uheit higher than the melting point, can be ascertained in.III!lIISways. Sheppard 6 describes one method which is stated

I•• rivc re ults of the accuracy commonly required in emulsion111.1lll1g,which is as follows: Equal amounts of the solutions toIII I('<;1cd are placed in similar test tubes of one-inch diameter.1111' (' are then cooled in ice water and are examined at definite11111'lvals,until the approach of solidification is indicated by the111I Ihat the solution is hardly disturbed when the tube is11111'" When this stage is reached, the tubes are inverted atI11IJI11'11I. short intervals. When the meniscus of the inverted

11111'11'110 longer sags, a thermometer is thrust into the jelly


and the stationary temperature is taken as the setting point.The rate of setting is, of course, of considerable importancewhen emulsions are to be coated on any kind of machine in thefactory. Generally speaking, it may be said that the rate ofsetting is reasonably gauged by the viscosity, or the jellystrength." While the rate of setting and the setting point canbe controlled to a large extent by additions of hardeners (alum,formalin, etc.) or alternatively by softeners (glycerin, etc.), ifa gelatin can be selected which resembles previous stock with-out alteration of the additions, it is far better. The use ofchrome alum, for example, is cumulative, and plates or filmscoated with an emulsion containing it will go on hardening forseveral months, during all of which time the rate of penetra-tion of the developer will be progressively retarded, with aconsequent alteration in the apparent speed unless carefullychecked.

It is very desirable in all tests of the physical characteristicsof gelatins to submit the sample to the same initial treatment.Thus any change in pH will be responsible for wide variationsin the swelling capacity. The ions of many salts have a pro-found effect on physical properties. As an example of labora-tory practice, in making a five per cent solution,s grams shouldbe weighed out and placed in small pieces in 90 cc of distilledwater (or tap water if the test demands), and allowed to swellfor one hour at room temperature, 700 F. The water is thenwarmed, the gelatin dissolved by stirring, and the volumebrought up to 100 cc. The temperature should not be allowedto exceed 1600 F. It should then be stood on the benchand allowed to cool until it attains the temperature that is re-quired for the test, for which a suitable temperature wouldbe 1050 F.

An example of a commercial test is tabulated on the nextpage.

MATERIALS FOR EMULSION MAKING 29

GELATl TEST. Lot No. II7.(;rcase .Clarity .En!(ler viscosity at IO%, temp. 35° C .Selling point of IO'70 solution .Melting point of IO% solution, after two hours .Reduction by ammonia-silver nitrate .Copper, lead, zinc .I ron salts .p H .

10. of cc of 50/. chrome alum required to precipitateIO cc of a IO% solution at 50° C .

oneComplete

3·327.8° C.3I.9° C.

oneloneTraces

5.6

5·9

For more detailed information on gelatin, the reader is referred10 Dr. S. E. Sheppard's monograph" Gelatin," Vol. I; Kiss-ling's" Leim und Gelatin" (Stuttgart, 1923); and the text-hooks of Alexander, Bogue, and others.

The brands of gelatin largely used in emulsion making areNelson's No. 2 and Nelson's X-opaque and S. E. emulsion leaf,(made at Warwick, England), Simeon's Winterthur (Swiss),l lr-inrich's and D.G.F. (German), Bertrand's (France), andInst, but not least, the excellent products of the Atlantic Gel-,tt ill Co. and American Agricultural Co. Most makers are gen-t'l'OlISin the extreme with regard to supplying samples, but itI a useful guide to them when ordering to specify the approx-1tl1:1t viscosity and hardness desired, if possible to send samplesIII t hose brands in use, 'and to state the type of emulsion forwhich the gelatin is wanted - fast negative, slow positive,lunmide or chloride paper, and so on. Prices average aboutH cents to $1.25 per pound, and as an example of likely quanti-lit's for commercial work on a moderate scale, about twenty-IIV(' to thirty pounds may be estimated for each day's run of\ ,000 feet of sensitized material forty-two inches wide. Gel-

,11 ill will keep almost indefinitely if stored in a cool and dry1'111('(',and it is the experience of many firms that if kept for


six months before use it gives the best results. As alreadystated, bulk gelatins are best made up of mixtures of two orthree batches or makes, which are arranged to run out at dif-ferent times. A couple of weeks should be allowed to make athorough test of a new sample. In making the test, a word ofcaution may be given. In packaging leaf gelatin it is the manu-facturers' usual practice to draw from two or three bins so that,a one-pound packet may contain three or more different va-rieties. If fifty grams are required from a pound packet, a crosssection of sufficient thickness should be cut on the cutter sothat a piece of each sheet is included in the test solution.

The reader ma~ wonder why gelatin remains the only col-loid which is successfully employed as a vehicle for sensitivesilver salts. Its sensitizing properties are sufficient reason, butits vehicular power, porosity and protective powers combineto make it unique. Collodion is still used in process work, andfor special types of emulsion of a slow speed. Silver halidescan be precipitated in a solution of pyroxylin in alcohol-ether,and collodio-brornide and collodio-chloride papers have longbeen in commercial use. They possess one advantage, that ifcoated on a base first coated with gelatin, the developed imagein collodion can be removed or stripped off the support bytreating the print with hot water, which dissolves the gelatininterleaf but does not affect the collodion. An example of thisis the Defender Chromatone bromide paper. The image, aftertoning, can be floated off the support and mounted in layersfor the production of a three-color print. Stripping papers canbe similarly made for a variety of purposes, although it is adoubtful point whether a coating of hardened gelatin emulsionon top of a gum dammar or wax coating of the support doesnot yield a better material.

We have seen in Chapter I how excessively small need bethe proportions of ingredients or impurities in gelatin to be

MATERIALS FOR EMULSION MAKING

responsible for important changes in emulsion quality. It istherefore easy to understand that all chemicals used in emul-sion making should be of a high order of purity. There arethree types of chemicals generally available. One is the stand-ard type sold for amateur and professional photography. An-other is the so-called chemically pure or C.P. variety, and athird type is old for the laboratory for more exact work, de-scribed by the term analytical reagent or A.R. For emulsionmaking in small quantities, A.R. chemicals are worth the extracost. At least silver nitrate and ammonium bromide shouldbe A.R. quality. In general, other chemicals may be of the c.P.type. In all formulas given in the following pages, water maybe taken to mean distilled water unless otherwise stated. Emul-sions are usually washed to best advantage in tap water, orsupplies from artesian wells.

Of the ordinary emulsion-making ingredients, ammoniumbromide is one of the most important. For extreme speedemulsions this should not contain more than 0.2 per cent ofchloride. The finest quality is probably made by combiningpurified bromine, free from chlorine or hydrobromic acid, withpure ammonia liquor. Less pure samples can be improved byre-crystallization. The chloride content can be checked byboiling a weighed sample with concentrated pure nitric acid,driving out the bromine, and titrating the residue with centi-normal silver nitrate solution.

Silver nitrate is made by dissolving silver metal free fromcopper and lead in nitric acid. The carefully re-cry tallizedproduct is usually sold at a price calculated on the ruling priceof silver at the time of order. It must be free from nitrite. Itsformula is Ag 03; that is, it is a salt without water of crystal-lization; molecular weight 170. For convenience, it may bekept in the form of a concentrated solution, such as 2 or2~ , in stone jars. In the following text, however, many

PHOTOGRAPHIC EMULSIO TECH IQUE

formulas will be found in which the silver nitrate is " wetted"with a small quantity only of water in order to convert it intoa highly concentrated solution of the ammonio-nitrate. Herethe dry salt will in most cases be found more conv.enient.

There is much to be said in favor of keeping chemical stocksin liquid form. It is probably more accurate and definitelymore convenient. In the case of silver nitrate, if a 2N solutioncontaining 340 grams per liter is made up, the ammonium bro-mide may conveniently be made up also as 2 ,having 196 gramsper liter. Equal quantities then react completely. On theother hand, such a reagent as ammonium bromide, which isused in slight excess of the equivalent weight of silver, maybe found more convenient to handle in the form of a ten percent solution.

Ammonia must be free from iron and pyridine compounds.There is an advantage in breaking down the concentrated am-monia to 0.920 specific gravity, as at this strength each cubiccentimeter will almost exactly re-dissolve one gram of silvernitrate or be equivalent to one gram of AgNOa' It will befound a great convenience to have it delivered in this form, al-though the gravity must be checked from time to time. Am-monium and pota ium iodides are best made up in the form ofsolutions and kept in the dark when not in use. A discolorationon keeping, due to the liberation of some free iodine, need notbe feared. Indeed, some emulsion makers think that the solu-tion in this form is preferable. Chrome alum should be se-lected free from contamination by small orange crystals ofdichromate, and should be made up with water not aboveroo? F., as otherwise it may decompose and lose its hardeningproperties: It should not be neutralized. Chrome alum is mostusually handled in the form of a five or ten per cent solution,which should be re-filtered if at any time, on keeping, it showsa deposit.

MATERIALS FOR EMULSIO MAlO G . 33

It is assumed that a reasonably accurate balance or pair ofs ales is available, and a larger balance where works-scale quan-tities are to be weighed. A balance sensitive to one or two milli-grams is needed, since in many emulsion formulas the reactingcompounds are used in proportions closely approximating totheir combining weights. The type of chemical balance which

FIG. 7

depends on a moving weight sliding over a scaled bar is notrecommended, but a balance of the laboratory type shown inFig. 7. A set of weights ranging from one gram to five hundredgrams with fractions to ten milligrams is suggested.

The following is a list of the chief chemicals which will beneeded in the course of the work described in this book:

AcetoneAcid, acetic, glacialAcid, citricAcid, hydrobromicAcid, hydrochloric


Acid, nitricAcid, phosphoricAcid, sulphuricAlcohol, denatured, photographic qualityAlcohol, pureAlum, chromeAmmonia, concentratedAmmonium bromideAmmonium chlorideAmmonium iodideBarium chlorideCalcium chlorideCarbitol acetateCupric chlorideDextrinDistilled waterFormalin, 40%Gelatins, variousGlycerinLitmus paperMethyl alcoholpH indicatorsNessler reagentPhenol crystalsPhenol ph thaleinPotassium bromidePotassium chloridePotassium citratePotassium dichromatePotassium iodidePotassium thiocyanateRochelle salt (sodium potassium tartrate)Sodium chlorideSodi urn citrateSodium oleate (glycerin substitute)

It is assumed that the usual photographic chemicals are avail-able. Calcium chloride or sulphuric acid will be required forthe desiccator.

MATERIALS FOR EMULSIOI MAKING 35A word may be said here about alcohol. Denatured spirit

to be used in emulsions must be free from mineral oil, aldehydes,pyridine or any colaring matter. A special type cif industrialspirit is made for the photographic industry which consists ofethyl alcohol (C2H50H), with a small percentage of wood spiritor methyl alcohol (CH30H). This, however, can only be ob-tained with a permit. A form of denatured alcohol containingabout one per cent of gasoline which has been used by theauthor with most types of emulsion with succes , can be ob-tained without a permit in any quantity. This is Synasol, sup-plied by Carbide and Carbon Chemicals Corporation, 30 E.42nd Street, ew York City. Where emulsions are to be usedIor commercial purposes it would not be recommended, but forexperimental work it may be found a great convenience. Whereonly small quantities of spirit are likely to be used for laboratoryscale experiments and cost is of no object, the reader is advisedto use pure grain alcohol or ninety per cent alcohol, both ofwhich are available in most laboratories. In work practice, apermit will be required for the necessary amount of industrialspirit, which will run into considerable quantities. Spirit isu ed in commercial emulsions in proportions varying from fiveto ten per cent of the total volume. In factory routine, frequentchecking up of the spirit is necessary to look out for the presenceof aldehydes or other deleterious impurities.

While a still for the preparation of sufficient quantities ofpure water is an essential on any works-scale production, whatis more important in selecting a site for an emulsion laboratoryi to determine whether or not the available water supply issuited to the work.

Chapter References

I. R. H. Bogue, The Chemistry and Technology of Gelatin and Glue,P·571.

36 PHOTOGRAPHIC EMULSIOl TECHNIQUE

2. M. Huzii, Reports Imp. lnd. Res. Inst; Osaka, I8, I, 1937.3. B. V. Storr, erui Ann. 'Report, Soc. Chem. Lnd., 37, 465, 1918.4. C. Stelling, Chem, Zeit., 20, 461, 1896.5· Kopke, Arb . Kais. Gesund., 38, 290, 1911.6. S. E. Sheppard, Photographic Gelatin, p. 203.7. Bogue, The Chemistry of Gelatin and Glue, p. 416.

CHAPTER IIILABORATORY EQUIPMENT

Layout for Experimental Work - Commercial Production and Its Re-quirements - Ventilation - Safelights - Digesting Apparatus - Therrno-talic Control- Washing and Filtering of Emulsions - Making up-

Cold Storage

rrHE accommodation required for emulsion making dependsprimarily on the quantities involved and type or types of

product to be made. As already hinted, one of the first thingsIII be ascertained in contemplating commercial production is thesuitability of the water supply or the provision of good water,nnd freedom from atmospheric contamination. While mod-('1'1l air-conditioning plants make it possible to work in towns.md cities, a new plant is generally started in a vicinity whereI he' air is clean and fresh and space is plentiful.

We shall endeavor in this chapter to discuss plant and ap-paratus both for the experimentalist and for the production of( nmmercial quantities. In many instances, of course, the largerplant is a multiplication of the smaller unit. But where the(lIating is to be done by any type of machine, conditions arelU'ccssarily very different from experimental work where the(lIilling is by hand. The laboratory type of work will be dealtwith first, but it will be understood that much of what is said.ipplies to both cases.

While small quantities of emulsion of excellent quality can1)('made in the laboratory, even in speedy varieties, the emulsionmnker must have the right tools to work with, light-tight.11 commodation, reasonably good ventilation, an ample water. upply, and above all, a room free from dust for coating, whereIII':I( up to 70° or 75° F. can be more or less controlled. Very

PHOTOGRAPHIC EMULSIOl TECH IQUE

small scale work can be done in one room, used alternativelyfor mixing, washing, 'coating and testing. In many instancesof experimental work this is the only condition under which itcan be carried on. The ideal accommodation for small-scalework however is to have at least four rooms or subdivisions of, ,a large room, where (a) the solutions can be weizhed out andprepared, (b) the emulsification, ripening, washing and digest-ing can be carried out, (c) the finished emulsion can be coatedand dried, and (d) the final product can be tested or used forphotographic work. The latter is obviously the average photog-rapher's darkroom.

The coating and drying room should be as free from dust aspossible, with sufficient and suitable ventilation which involvesno light leaks. Cleanliness and order are above all things tobe desired, the utmost care being taken to keep all graduates,crocks, stirring rods, beakers, dishes, thermometers, etc. scrupu-lously clean. Cupboards are better than shelves for storingchemicals and apparatus, bottles and glassware being first classdust catchers and requiring frequent rinsing and wiping witha clean damp cloth.

Any room to be used for coating and drying sensitive materialmust be thoroughly light-tight, this usually involving rathermore investigation than would suffice in ordinary darkroomwork. By shutting oneself in the room in complete" darkness ))for ten or fifteen minutes, it is often astonishing to find howmuch stray light there is in a room previously thought to beabsolutely dark. Black or dark brown paper and paste or glueis generally a sufficient remedy, or the filling up of cracks withmortar or plaster of Pari. A strip of felt or rubber beadingalong the bottom of doors is often a help, or a wooden lathscrewed to the floor to cover the clearance space. Benches aboutthree feet high are more convenient than tables, as practicallyall work must be done standing and not sitting. This does not

LABORATORY EQUIPMENT 39

T

E/YJUJ..SIFICI'ITIOrvliNt;)

RIPENING-

WEICTH'II/(i- - LIP

""Nf)c:.IYEII1/C""J.

~QQrI')

COATIN,"r-AND

ORYING-

TW,</SHINCr

~NDPIIV/ISHINw

TE~TING-~OON}

1----H ----11

FIG. g

'\llpl y 10 benches with sinks for washing jar and crocks, whereI 111111t'convenient height is two feet to two feet, three inches.1111 should be of ample size, and if of porcelain or enamelled

111111;J nrl not of wood, wooden slats should be placed over the1111111111110 prevent breakage.

\ mall Iayout is shown in Fig. 8, in diagrammatic form. TheI 11I'I,didea, as is often suggested for commercial laboratories,

I IIi.ll 1he work should take place in a cycle, one operation fol-1'1\ Ill' another in the ordinary sequence, so that chemicals and11111111ms are prepared at one end, and the testing of the finished

1'llIdll( I ran be carried out at the other. It is a great conven-111111'III have a light trap between the coating room and the1111111·IlIlling room, as shown at T in Fig. 8, as one can then go1111h 11'11111"light " to " dark)) at will. If two people are doingIhI \ IIIk , fl small hatch 11 with sliding panel is useful for hall-

PHOTOGRAPHIC EMULSIO TECHNIQUE

dling crocks, solutions, etc., from the making-up room to thecoating room.

A word may well be said here about illumination. It is acommon idea to have black walls in the darkrporns. This isbad practice. If the source of light is properly filtered, the wallsand ceilings may be white, when they will reflect a maximum

SLOW BROMIDERESPONSE

TRANSMISSION OFYELL.OW SAFELfGH T

FAST NEGATIVERESPONSE

TRANSMISSION OFREO SAFEL.lGHT

ORTHOCHRO/t1ATICRESPONSE

TRANSMS'N OFRuBY SAF£LlGHT

UPPER: PAIv'CHROMA T/C RESPONSELOWER: GREE.N SAFELlGHT TRIlNSMSiv

1=00 soo 600 700 rnf'

FIG. 9

LABORATORY EQUIPME. T 41

amount of diffused light from all parts and greatly increase com-fort in working. Inverted ceiling lamps throwing the light up-wards so that no direct rays fall on the bench or table are thebest to use for general illumination. The ordinary vertical typeof lamp will be needed on the benches themselves. Electrictorches provided with a piece of " safe" gelatin or zlass fittedin the top should be provided for reading thermometers in thedarkroom.

Type of emulsionChloride and chlorobromide

Calor ofsajelight

Yellow

Panchromatic .

TypeWratten 00Agfa 105

Wratten 0 or OAAgfa 104

W rat ten Series 1

o I' Series VIaW rattcn Series 2

or Agfa 107

Narrow-cut Wrattcn Series 3green or Agfa 108

Same as for panchromatic

Bromide paper or transparency ... Orange

Non-color-sensitized Red or yellow-green

Orthochromatic Deep reel

Infrared .

The spectral transmission of these various types of safelightis shown in Fig. 9, with the average spectral response of typesof emulsions for which they are recommended shown immedi-ately above them. Ground-glass bulbs are to be preferred toplain glass as they diffuse the light better, unless the safelightglasses themselves are of the diffusing type. Ten-watt bulbs areobtainable from most photographic dealers and should be usedin the bench lamps.

As white light must also be provided in all rooms, it i a goodplan to place the switches for them high up at the side of thecloors, so that a physical effort must be made to reach them.This simple arrangement almost entirely prevents turning onIhe white light by accident. In any works operations the need


for proper light traps. between darkrooms is imperative, andautomatic switches are usually provided for making it impos-sible for anyone to turn on the white light when the safeJightis being used. The white lights in the emulsion and coatingrooms should be of low candle power in order to keep the eye-sight for dark operations. It is customary in modern coatingand cutting rooms for operatives to spend a few minutes in apreparatory room of very low illumination before starting work,and to rest there for a short time after work to accustom theeyes to the change of level.

As regards apparatus, we shall again consider the experimen-tal requirements first, discussing the commercial plant later. Itis hardly necessary to say that for what apparatu may not beavailable, simple substitutes can usually be found. On the otherhand, it is more likely that additional apparatus will be addedif any serious work is undertaken, such as an accurate balance,filter pump, viscosimeter, proper temperature control, somemeans of pH determination, .etc. In connection with thermom-eters, the advice is given tospend liberally on good quality onesthat can be easily read. Except with cc boiled" emulsions, tem-peratures above 1400 F. (600 C.) are rarely needed, and if ashort-scale type be used, the readings will be cone pondinglyclearer in the darkroom. Eimer and Amend ( ew York) makea student's type 30 cm long, reading from +100 to +IIOo C.and +20

0 to 2200 F., marked either on the glass stem or on

a paper scale inside the outer glass. Both types can be obtainedwith a short scale reading from 00 F. to 1300 F., which is a greatconvenience. [ote that chemical thermometers scaled for totalimmersion are not suitable for general work. Thermometers ofthe clinical type, with a broken thread of mercury so that a tem-perature taken will remain recorded until the mercury is shakendown, are very useful. These can be obtained with any tem-perature range if specially ordered through a maker or scien-

LABORATORY EQUIPMENT 43

1111Iapparatus dealer. It is often recommended that the bulbIlIlIilcIbe protected by a perforated gauze of some non-reacting

111I'1.iI,but probably the best way to safeguard thermometers inI urulxion work is not to use them as stirring rods!

1',I\1ulsionsare ordinarily made in stoneware or earthenwareI 1111I s, and these are not easy to obtain in small sizes except1111111scientific apparatus makers and some of the larger depart-1111'111stores, Glazed stoneware jars, sometimes used as cc waste[.u "in laboratories, can be obtained from Eimer and AmendIII il.(,s from I gallon capacity upwards, this size being 7~ incheshlvh and 7~ inches outside diameter, and costing, with loose1IIIII'warecover, ju t over a dollar. The experimenter will, how-

I \ 1'1, require smaller sizes, and such crocks, with covers, of one1'1111,one quart, and half-gallon capacity should be obtained.1'111very small mixings there is nothing so convenient as a small\ r-ll glazed earthenware casserole pot of the tall, not squat,1'111'. These can be had on searching diligently enough at someIII t lu: hardware and department stores. At least one largeIIIIII'ware jar 'with hole or cc tubulure" at bottom must be avail-d "I' for washing emulsions (see p. 70). A one-gallon jar will1111111'for washing any negative emulsion that is to finish up

11111IIIOI'Cthan one liter (35 oz.); a two-gallon jar will suffice1111lilts of two to three liters. Rubber corks should be used for111111I11lgS,but they should be washed with hot soda water be-1.11111('

(:1.1S stirring rods should be twelve inches long and at least11111h,df inch in diameter. A stout stirrer that can be grippedtu mlv so that good wrist action can be applied, which is stoutI 11""VIito beat up the emulsion thoroughly, is quite an impor-11111111'111. tirring rods with rubber guards should not be used.

, I Ill'S and weights must be reasonably accurate, as such111.111quantities as fractions of a gram have frequently to beIlplll'l!. Some sixteen-ounce and thirty-two-ounce bottles are


useful for stock solutions. Graduates ranging from ten cc to500 cc should be available, and some Pyrex or glass beakers of100 and 250 cc capacity. Pyrex beakers, student's pattern,graduated in clear figures, can now be obtaine which are al-most unbreakable, cheap, and approximately accurate for gen-eral work. One or two plain filters, thirteen to fifteen cm out-side diameter (250 to 500 ml capacity) will be needed, and acouple of separatory funnels if mixed jet emulsions (p. 100) areto be made.

As most solutions are used hot, it is important to have suf-ficient heating facilities for getting the various items ready foremulsification. Depending on the size of the operation, any-thing from a large aluminum saucepan to a steam-heated rec-tangular wooden or stainless steel tub with heavy service im-mersion heater can be used. A metal heating pan about sixteenby twenty inches and about eight inches deep is a very conven-ient size for experimental work, with either an immersion heateror a small gas ring underneath. Either can be easily thermostati-cally controlled. For small single lots of emulsion, an aluminumsaucepan standing on an ordinary domestic type of electricheater will answer quite well. In many laboratories, exi tingequipment will meet the case, provided it can be kept sufficientlyfree from contaminating influences.

A good laboratory stop-watch is invaluable if any seriouswork is intended. As important as the watch is a really gooddarkroom clock, which can be depended on to ring the alarmbell 'when a pre-set time has expired. A little extra money allo-cated to the clock will be found a good investment. In prefer-ence to the use of luminous hands and dials, it is recommendedto place the clock on a small box fitted to the wall, a strip ofdark green safelight being inserted in the top (Fig. la) throughwhich light from a ten-watt lamp at the bottom of the box passesupwards on to the dial and makes it just sufficiently visible. A

LABORATORY EQ IPMENT 45

111;111 projecting piece, P, will prevent any light spreading toI lu- room. A metronome is exceedingly useful for counting sec-IlIld when working in complete darkness or very feeble pan-I III umatic light.

p--11£:iml'11foff----~"

FIG. 10

I r Ihe student of emulsion technique wishes to study grain-uiwth and morphology, a first-class microscope outfit will be

11 quircd. A low power is useful for studying mechanical de-11 I I and large developed grains and grain aggregates, bu t for111\ rxamination of silver halide crystals a n.-inch oil im-1111"1 ion objective is needed. Photomicrography of silver bro-111111" cry tals can be done with surprising success without the11 I' Ilf any photomicrographic apparatus except a ninety-degreeIII I III on the eyepiece. This simple technique has many ad-

1111.1j.(('S over elaborate equipment. The microscope, with its11111 I Ill" r moved, is placed on top of a light-tight box, with anq 11 11111"(' about an inch and a half in diameter, H, (Fig. II) cut

11111 III the top. A lamp L (Pointolite or concentrated filament

PHOTOGRAPHIC EMULSION TECHN IQUB

type) is placed underneath, in tbe box. The rays thus pass di-rectly upwards to the Abbe condenser. The small eyepieceprism, P, deflects the rays at rigbt angles on to the wall, wherea plate, AB, is stood on a suitable ledge or support, and receivesthe image. Final focusing of the crystals is thus done actually

..'A

FIG. II

on the plate, and all that has then to be done i to switch offthe lamp (or insert a filter in the light path), replace the plateAB by an unexposed one, and make the exposure. When theeyes get accustomed to tbe complete darkness in which this workmust be carried out, exact focusing can be done quite easily. Abit of dead-blacked brass or a 'cardboard tube between the aper-ture in the top of the box and the Abbe condenser will preventany extraneous light from 'fogging the plates. As with highpower an image of the light source should cover the whole field,a good alternative to the Pointolite is one of the flat strip in-

LABORATORY EQUIPlVIEr T 47

Id ndcscent lamps used in sound recording, which runs from aI volt storage battery taking about thirty amperes. These

I.IIIIPS are obtainable from the General Electric Company.While, for cooking or digesting purposes, Thermos or Dewar

ILI"ks are most convenient for small lots of emulsion, a thermo-I.d ically controlled pan is more or less a necessity if work on any1'1 ious scale is to be undertaken. A metal pan similar to that

,tln'ndy described for heating solutions will be convenient, butI i( will be required at a lower temperature it should be a sepa-

1.11 (' piece of apparatus. A deep ish pan sixteen by twenty1111 ill'S and twelve inches high is a convenient size. Thi canIll' I Cl t filled with water to a height of five or ix inches, or\\ iIhin an inch and a half or two inches of the top of the smallestI II)('k used. For uniformity of work, standard-sized lots ofr-mulsion should be made as far as is possible, so that the waterI('wl in its relation to the amount of emulsion is kept fairly con-.I :1111. In other words it would be bad practice to have a half-rnllon crock eight inches high standing in only three inches ofwater, even if the contents were kept well stirred.

Such a metal pan can be fitted with a hinged lid unless ther-mul ions are to be stirred during digestion. In some cornmer-I I:t! plants, where very long dige tions are given at a low tern-11('l'ature, it has been found best to digest or finish without stir-I illg, the whole of the bulk gelatin havinz been added anddissolved beforehand so that sedimentation is reduced to a min-unum. Any coarse grains which do settle can be largely elim-mnted by careful decantation or suitable filtering. On the other1I1IIld, Carroll and Hubbard state 1 that in their experiments alli-mulsions were stirred continuously during digestion to prevent. ('dimentation. Similarly Trivelli and Sheppard state 2 that" Ih object being to obtain a uniform, relatively fine-grainedmaterial, silver halide emulsions are continuously and thor-uughly stirred." With short-timed finishing of very high-speed

PHOTOGRAPHIC EMULSION TECH JIQUE

emulsions having a long scale of gradation, it is the author's~xperience that the crock should be left unstirred in the digest-ing pan, but every formula has its own particular optimum con-ditions and no fixed rule can be even suggested.

The apparatus needed for washing small lots of emulsion willdepend entirely on the bulk that is to be handled. In a numberof the formulas given in this book, a small unit of about oneliter is set out, which can be multiplied as required for com-mercial bulks. The bulks used in manufacture, as elsewherestated, may run up to fifty or sixty gallons in the case of paperemulsions, while fast emulsions are generally made in smalleru.nits which are blended for better uniformity. It may be per-tinent to refer here to what appears to be a very indiscriminatemixture of grams and ounces, liters and gallons, centimeters andinches, Fahrenheit and Centigrade. The metric and avoirdupoissystems have become inextricably mixed in the industry, andany reference to photographic literature indicates that the twosystems are so much in use .that alternative figures have to begiven. The gelatin manufacturer sells his gelatin by the pound,rarely by the kilo, even in Europe; dyes are sold by the poundand in ten-gram or one-hundred-gram lots; i1ver nitrate isbought largely in rooo-ounce parcels in England, but is meticu-lously dissolved up in the factory in the form of hi-normal ortri-normal solutions which are measured by metric system foruse. And so it goes on. In most case temperatures have beengiven in this book in both Centigrade and Fahrenheit figures.

To return to the wa hing apparatus. The stoneware jars re-ferred to on page 43 will be found ideal for washing small lotsbu t as explained later tanks are used for commercial batches,As good washing depends on getting the noodles in as intimatecontact with the water as possible, their gentle agitation in eachchange is necessary. The agitation can best be done by gentlestirring with a glass rod or spoon, keeping the shreds well sepa-

LABORATORY EQUIPMENT 49I 111'<1without breaking them. At the end of each change, the\ .rtr-r is run off, provision being made that no shreds escape1IIIIHlghthe outlet. A false bottom of suitable textile material111.1\ be used, so that when the wash water is run off the noodles'.Ill he thoroughly drained. When washing is complete, drain-Ill' should be made as complete as possible. It sometimes hap-III'IISin washing that the gelatin takes up so much water that1111remelting the volume is greater than the desired amount.I Ills will lead to thinne of image and frequently to fog. Theumcdy is to use colder washing water, to use a less absorbent",,1.11in, or to decrease slightly the bulk of the reacting solutionsI11mixing. It is good practice in the laboratory in hot weatherIII throw in a few ice cubes with each chanze of water if theu-mpcrature of the darkroom cannot be controlled.

11will be noted in the formulas which follow that about fivepl'r cent of alcohol is included in the" finals." It will be foundpllnd practice to pour this alcohol on the washed noodles afterIIi('y have been transferred to the crock, and before starting to111I'1tUI. This accelerates melting, which is good practice, asI1confines the final digestion or finishing within stricter control.The crock should therefore be stood in really hot water, r600

Ill"1800 F. and the emulsion kept well stirred, so that the shreds111'01re t the outside of the jar do not get raised to a higher tern-pl'I"OIturethan those in the middle. When the emulsion tempera-lurr has reached roso F. (400 C.), it should be removed fromIII(' water, because the jar itself will be hotter, and a furtherII~t' in temperature of the contents will take place. Great careruuxl be taken, in fact, that the temperature of the emul ion is11111allowed to exceed that of the final digestion temperature,.111(1that as soon as this has been reached, the crock is trans-h-rrcd immediately to the digesting pan. When the digestionj, linished, the emulsion should be poured into a cold crock andIht' finals added. These are usually chrome alum solution to


harden the film, free ammonium or potassium bromide to pre-vent edge-fog in platesand fog on keeping (in the case of filmsand papers), a preservative such as carbolic acid, thymol,sodium salicylate, etc., and any such gelatin-plasticizer asglycerin.

The made-up emulsion is now filtered, and for laboratoryscale operations nothing better can be recommended than twoto four thicknesses of good quality cambric or linen which hasbeen boiled in order to remove any sizing. A fine quality men'shandkerchief is excellent for the purpose. Filter paper is use-less; chamois leather is not advisable, as it is u ually unevenin thickness and the emulsion merely runs through the thinnestpart. If suction is available, such as that from a good filterpump attached to a faucet having a good head of water, a Buch-ner funnel will be found very useful. It may be' questionedwhy, when chemicals of the highest purity are employed, anyfiltration should be needed. Leaf gelatin contains a good dealof string fiber, unless dried on wire nets; if ground, it will con-tain a surprising amount of dust and dirt. Even re-crystallizedchemicals contain some dirt, and an examination of the filtercloth, after the emu I ion has run through it, will reveal the needfor filtration. In addition to mechanical dirt, filtration goes along way towards breaking up clumps or aggregates of silverhalide grains, which are always formed to a more or less extentin the process of making.

If the cambric or linen is laid over the top of a clean crock, itshould be attached by a piece of twine or tape tied tightlyaround the top edge. The linen is gently pressed downwardsto form a bag, and the emulsion is poured into the bag, andfiltering helped with a glass stirring rod. If the emulsion isobdurate, a vacuum can be created in the crock by pulling thelinen tight with the hands; this has the effect of drawing theemulsion through. It is the usual practice to coat a test plate

LABORATORY EQ IPiVIE T 5I

,11 lhis stage, and then to set the emulsion off in ice water, re-nu-lting it again a day or so later when required for coating.1';lllul ions will usually keep for several days, or even weeks, ifIored in a room kept below 450 F.

We can now give some attention to work on a larger scale,where somewhat different handling of the emulsion will be re-quired. While the" unit" lots described in subsequent chap-u-rs may be increased by simple multiplication, as the bulk'I'OWSlarger various modifications must be made in order toprevent overheating. One of the most successful pioneer dry-pia le manufacturers always claimed that it is possible to pro-t111l'C in one liter (35 oz.) all the qualities and speed of a works-i;~('batch of emulsion, and this is correct provided that works

hutches are made in moderate units, such as two to three gal-lnns and not in bulks of fifty to a hundred gallons. When plan-11iIlg works-scale operations, it is advisable to decide on thepolicy to be adopted; that is, whether to make in comparativelyIlia11lots of two or three gallons and mix these at a later stage

1111 coating, or to manufacture in bulks of five or ten times this'iz('. The difference in sheer mechanical handling that is in-vulved must be obvious. Convenience in handling is muchfll'('ater if moderate quantities are made at the time of emulsi-urntion, and blended after washing. The shreds can be mixed.ritcr washing, and rernelted as a blend, or each small unit canIll' carried through to the end and tested bejore blending, whichI a surer way of obtaining perfect uniformity of product.

In the commercial manufacture of films, plates, and papers,IWIIthings of first importance stand out - freedom from dust,,dready discussed, and adequate control of humidity and tern-IlI'ralure. It unfortunately happens in many cases that a build-IlIg designed for some other purpose is adapted for the makingIIr s n itized products. Such a building is rarely as efficient or, :1 Iisfactory as one which has been specially designed. On the


other hand, many factories which the author has visited in Eu-rope show signs of continual additions due to trade expansion,where compromises have clearly had to be made. One note-worthy difference between a plate factory and one for film orpaper coating, is in the length of building required. Whileplates will set in half a minute on a thirty-foot machine, andcan be lifted off and stacked in cupboards to dry, both film andpaper have ordinarily to be run from the coating head to a dry-ing tunnel where the material hangs in loops or festoons andtravels slowly along the track for anything from three hundredto six hundred feet. Being dry by the time it reaches the farend, it can be immediately re-reeled, thus making the processa continuous one, which is frequently carried on three eight-hourshifts a day for days at a time. Koebig's catalogue 3 gives par-ticulars of a film coating machine which winds the coated stockin the manner of a spiral, thus greatly conserving space.

Whatever the product may be, we are concerned in this chap-ter more especially with the emulsion-making department.Here the importance of cleanliness must be again emphasized.Ample space is most desirable. Temperature control is indis-pensable. The layout already discussed for small-scale opera-tions needs merely to be amplified and elaborated. In manyfactories, the work is distributed among several buildings, eachcomplete in itself. Each one has then its own emulsion-makinglaboratory to feed the particular coating room. This system, bythe way, has the great advantage that if, as occasionally hap-pens, some chemical or other trouble arises, it hampers workin only one section of the factory.

If the whole laboratory layout can be on one floor, so much thebetter, as then the various rooms can be arranged in definite se-quence, so that stores and weighing rooms are the commence-ment, and the finished emulsion comes out at the end. The storefor chemicals and gelatin should be arranged so as to be conven-

LABORATORY EQUIPMENT 53EMULSION LABORATORY (DOORS AND LIGHT TRAPS NOT S/fOWJY)

STORE

~ CY6sc)WEIGHING- ,

" \!) ItUP ~ ~

~ I> ~• ~.. "- '<! '"~ '"ROOM '"~ ~ :;:~~ 1::~

PASSAGE'W.IIY-=NE.AT/NCf UP

J..ABORATORY

AND

OFFICEs ORE

BENCH DJ a e s r r or-:

fIG. 12

1'111Iy situated for unloading goods. The finished emulsion canIll' wheeled in small- light-tight trucks from the making-up roomIII Ih building where the coating is done. A suggested plan is'1'1'11in Fig. 12, where the store leads into the weighing room,111111the crocks and silvers are heated for mixing in a small com-lIill'lment next to it. The emulsification is then carried out on1111'h nch in the next compartment, the emulsion placed in thewn1'111water pans to ripen, and then set in the cold tanks in1111'cnd room. It will be noticed that in the emulsifying roomIWIIril ening pans are shown. In many emulsions of the" all-1111I110nia"type, emulsification is carried out at a low tempera-illl'(' and when the bulk gelatin is added, the emulsion must bel,ris('c1to about lO5° F. as quickly as possible, in order that itIlIay be properly dissolved. It is often necessary, therefore, to


have one pan for ripening at the lower temperature and a hotpan for heating the jar after the gelatin addition. With largebatches of emulsion, ome form of mechanical stirrer is a con-venience in the latter operation.

The term" ice water," used in the setting troughs, is verycommon in emulsion parlance, but is unfortunately a somewhatslipshod one. It may be taken to mean, however, that the tem-perature is around 36° F., and tliat some ice is always keptfloating in the water. The cold pan water can of course becooled by a refrigerator coil and thermostatically controlled.

FIG. I3

The set emulsion, which may in some cases be washed in fromthree to five hours, though more generally the next day, ispressed into noodles or shreds in the setting-off room by thepresses indicated. The noodles are most conveniently washedin the same department. As considerable quantities of watermust be handled in the washing process, a cement or composition

LABORATORY EQUIPMEI T 55

llonr and walls which can be hosed down, with good gutters run-1IIIlg (0 an ample waste, should be provided. Large faucets areIll,t'decl to expedite the refilling of the washing troughs, and auitable water main with ample pressure will be needed to

III me the necessary hydraulic pressure for the shreddingm.rchines.

FIG. 14. FLOOR PRESS SHOWING EM LSIONCONTAINER SWU, G OUT FOR LOADING \VITH

JELLY

I'Il('SC pre ses may be attached to a wall fitted in a strong11'1'1 frame for suspension from the roof (Fig. I3) or can be

11 I'd in wooden frames on the floor. The emulsion containerI 11 11:1lly made so that it can be swung clear of the pressure cyl-Ildl'l' for charging, as shown in Fig. 14. This represents a

PHOTOGRAPHIC EMULSIO TECH IQUE

Dixon 4 press, which is constructed of gun metal, with a pistonrod of stainless steel, and a plunger cap of nickel fitted with awooden face. The emulsion container (seen swunz out forcharging in the figure), is lined with pure nick ,and the per-forated shredder plate, which drops into the bottom of the con-tainer, may be made of pure nickel also. This plate, having per-forations three-sixteenths or one-fourth inch in diameter, mustbe very stout so as to stand up to the force of the cold jelly whensqueezed through it. A sterling silver shredder plate is stronglyrecommended by the author.

The washing tubs, which receive the noodles as they fall fromthe press, may be of wood or stoneware. If the latter, they

£H

c; 1.- ....•FFIG. I$

should be provided with wooden trestles which can be wheeledeasily to the part of the room where washing takes place. Con-siderable divergence of opinion exists as to the best method ofwashing. If four volumes of water are allowed for each volumeof emulsion (before shredding) and the noodles are kept suf-ficiently separated by wooden paddles or some simple and gentleagitating device, and the wash water is thoroughly drained offafter each change, it will be found that twelve changes of five

LABORATORY EQUIPlVIE IT 57

minutes each, with three minutes draining, will adequately washprurtically any type of emulsion. Three or four hours are rec-1I111l11cndedby some writers, but this tends to over-swell thel'II1111sionand affect its character.

One form of washing trough is shown diagrammatically inI'ig. 15. Here an inner wooden tub, ABeD, with gauze or can-I.lS bottom drops into an outer trough, EFGH. The inner ves-1,1is about one-third filled with noodles, and water is fed into

I1until it well covers them. In draining, a small loss of emulsionI inevitable, as, in mechanically keeping the shreds separated,111:111bits are bound to be broken off and pass through the111'('11at the bottom of the tub. The loss in washing should

11111('X eed one to one and one-half per cent. Cold water, neverIdlllve 55° F., should be used.

The washed shreds, as explained later, are remelted and sub-1'1I('cl to a process of digestion during which the speed increases1'1 greatly, and while in most cases the temperature used lies

111'1wren 120° and 125° F. (about 49° to 52° C.), it may be con-Icll'rallly more in the case of slow emulsions, or of course of. huiled " emulsions (p. 6). A type of digesting pan which

I .m h{' made on a small laboratory scale or on a works scale is1'1'11in Fig. 16. The emulsions crocks stand in an inner rec-

1.111ular pan of metal which in turn stands in an outer one.11111h contain water. A thermostat shown in the inner pan con-I1Id, a fairly heavy heating element, via a suitable relay, whichI 11I'd at the bottom or at one side of the outer pan. If no stir-II"~' is to be given, the crocks may be covered with loose lids,11111ill many cases vertical stirrers are fitted to a loose cover on1111'Ilr Ihe pan, as shown in the diagram. These are motorcll"'I'II, and turn only fast enough to prevent sedimentation.

11111('ingenuity must be used to take care of condensation.!lI'1 dropping from the bearings of the stirrers or elsewhere

11111Ill(' rocks, which could easily fog the emulsion by chemical

S8 PHOTOGRAPHIC EMULSro - TECHNIQUE

FIG. 16

contamination. Some form of loose cover for the crocks them-selves as well as an overhead guard will combine to give thenecessary protection. Needless to say, the apparatus can besimplified by having one pan, only of water, the thermostat andheating element being contained in it along with the emulsion.It is then an advantage to have the outer water itself stirredto insure an even temperature.

The thermo-regulator should be capable of a range of controlof between r ro? and 140° F. (45° and 60° C.). An immersionelectric thermostat consists usually of a thin metal tube her-metically sealed at the bottom, which contains a sensitive bi-metallic bar that extends from the bottom about half-way upthe tube. The contact points are protected by a condenser toprevent sparking. The control temperature is often regulatedby a removable key so that, once set, it cannot be tamperedwith or altered accidentally. Care must be taken that thethermostat i capable of handling the load required, which willof course depend on the bulk of water involved.

Reference has been made in an earlier part of this chapterto Thermos flasks. For test lots of emulsion in the factory,

LABORATORY EQUIP'MENT 59

these are very handy and deserve some further notice. Thellask should be filled with hot water several minutes before itis needed, at a temperature two or three degrees higher thanth digestion temperature. If the emulsion when put into the\1ask is also a couple of degrees higher in temperature (Fahren-lu-it), it will probably drop to the desired point or a trifle belowduring digestion, the figure over the total time averaging outvery nicely. A practical point that should be emphasized is thatthermostatically controlled apparatus will operate most satis-I;(ctorily in a room where the temperature does not undergodrastic variations. Where the laboratory is maintained through-nut the year at about 70° F., very little difficulty will be found(11 controlling water baths and pans.

FIG. 17


The digested emulsion must be made up as quickly as pos-sible, and this is done by (a) lowering the temperature to1050 F. or below, and (b) adding such final additions as chrome

FIG. 18

alum, alcohol (if this has not been already added), potassiumor ammonium bromide, and any other chemicals. The speedthen stays put more or less, and the emulsion can be filtered andsent to the coating room or reset and put into cold storage untilrequired.

Filtering can be done by gravity, or with the help of eitherpressure or suction. A typical example of a pressure filter isthe Dixon model/ which is an enclosed cylindrical vessel of960 ounces capacity, fitted with a perforated metal plate for

LABORATORY EQUIPMENT 6r

the upport of the filtering medium, and an air valve for connec-t ion to a pressure pump. It is designed for use upon a metalor wooden table support, under which the emulsion jar or re-ceiver can be placed. It is seen opened out in Fig. I7 and closedfor use in Fig. 18. Two forms of suction filtering plant areseen in Fig. 19 and Fig. 20. In the first of these an invertedfunnel, over the top of which is tied the filter material, is placednear the bottom of the receiving jar. The tube of the funnel,.md a bent glass tube leading to a suction pump, fit tightly into;! rubber cork in the neck of the receiver. Moderate suction isnpplied to the bent tube. The other type is made by a well-I nown firm of earthenware manufacturers.' This is a jar withoutlet pipe for suction apparatus to be connected; a heavypl'rforated stoneware disc sits on top of the flange of the jar,uud above this is a stoneware top. The filtering medium isplaced between the disc and the top, and suction provides the('al. Another type of suction filter, made by Koebig," is seen

ill Fig. 21.

When filtering by gravity only, a larger apparatus of the typed('~cribed on p. 50 is used. The chief difficulty in all filteringIll' emulsions is the choice of material. Filter-cloth makers

__ > TOSUCTION

-------

FIG. I; FIG. 20


have a wide variety to choose from, but it will be difficultto improve on the four thicknesses of fine linen or cambric al-ready suggested.

FIG. 2I

The filtered emulsion is quickly set, and for this purpose itcan be returned to the setting tanks in the washing room, or

"-preferably to a separate cold room, where the temperature can beautomatically maintained at somewhere below 45° F., or the jarscan be stored in ice water or cool cupboards. The time thatmade-up emulsion can be safely stored, depends entirely uponits previous history.

Chapter Rejerences

1. Carrell and Hubbard, Bureau of Stands. Tourn, of Research, 7, 226,

I931.2. Trivelli and Sheppard, The Silver Grain of Bromide Emulsions, p. 32.3· August Koebig, Radebeul, Germany.4. Thomas Dixon and Sons, Ltd., Letchworth, England.5· Doulton and Co., Ltd., Lambeth, London, Eng.

CHAPTER IVJEGATIVE EMULSIONS

Their Structure and Compo ition - Types of Formula - Preparation ofI he Reacting Solutions - Emulsification - Ripening - Setting - Wash-IIlI: - Final Digestion or Finishing - Making up - Anti-halation Meth-lids - Reversal Emulsions

A EGATIVE type of emulsion of moderate speed is prob-ably the most suitable to select for a first consideration of

r-mulsion making. The speed and average grain size and some0\ her properties of the slower types of emulsion are as follows:

Relative Resolving SPeed AverageType Contrast Power Latitude * H & D Grain Size

"t. microns

'l'ransparencyor lantern 3.2

l'rocess 3.0l'hotcgravure 2.8

('lImmercial 2.2

Motion picturepositive 2.7 80 50 ISO 1-2

If< Ratio of exposure at limits of the straight line portion of the charac-u-ristic curve for development to gamma infinity. In many cases this por-111111 is measured between the points where gamma is 0.5 at the toe andhoulder.

roo 5908065

254075

1.520

I.0-240

90 1.5-2

In so-called grainless emulsions of the type described onpage 136, the mean diameter of the grains (AgEr crystals) is of\ 11('order of 0.0002 mm, or 0.2 ~t. While speed and grain sizeelo not necessarily increase side by side, the zrains of fast ernul-

iOI1 are generally larger than those of slower ones, reaching 3to 4 ~, although the diameters are invariably mixed. Thus

PHOTOGRAPHIC EMULSro TECH IQUE

Clerc 1 states that in. a particular emulsion the distributionbetween the different sizes has been found to be as follows:

Less than I J.I. • • • • • • • • • • • • • • • . . . . . .• 6 I per centFrom I J.I. to 2J.1. ••.• '" .••.....••••• 32 P centFrom 2 J.I. to 3 J.I. • • • • . • . • . • . . • • • • • . • 6 per centAbove 3 J.I. • . • • • . • . . . . . . . • . • . . . • . .• I per cent

In a slow emulsion for reproduction purposes the extreme ratioof diameters was found to be not more than one to five.

A number of rapid emulsions dried in air (containing an equi-librium amount of moisture of not more than IS per cent)showed on analysis to average:

Water .Gelatin .Silver bromide '..Silver iodide .

10 parts by weight55 parts by weight32.5 parts by weight

2·5 part by weigh t

100 parts

Traces of silver chloride were found. Weigert and Luhr statethat they found as much as 2 mg per square meter of free silvermetal. Silver chloride is almost impossible to exclude, as sam-ples of ammonium bromide may contain as much as two percent of chloride. A good sample of NH4Br for negative emul-sions should not contain more than 0.2 per cent. The slowertypes of emulsion being considered in this chapter would or-dinarily contain about half the above proportion of silver iodide,or less. As chemical analysis (p. 231) of a slow film indicatesa coating weight of some 60 to 90 mg of silver halide per squaredecimeter, the skeleton of the emulsion may be built up some-what as follows, it being assumed that a liter of emulsion wouldbe distributed over, say, I SO plates of 5 by 7 inches size, or atwenty-foot run of film base forty-two inches wide. For con-venience, the amounts of alkaline halides required to convertone gram or part of silver nitrate are given in the table whichfollows on the next page.

NEGATIVE EMULSro ~S 65

Rracting proportions of halides to combine with one part of si/vel' nitrate,AgNO. (molewlar weight I70)

mmonium bromide, lH4Br mol. wt. 98Potassium bromide, KBr mol. wt. II9

Ammonium chloride, H4 Cl mol. wt. 53·5l'otassium chloride, KCI mol. wt. 74·5Sodium chloride, NaCl mol. wt. 58·5Ammonium iodide, H4I mol. wt. 143PoLassium iodide, KI mol. wt. 166

reacting amt. 0.576reacting amt. 0.700reacting amt. 0.373reacting amt. 0.43 I

reacting amt. 0.408reacting amt. 0.617reacting amt. 0.638

Bearing in mind that the emulsion must be washed, duringwhich process the jelly takes up almost its own bulk of water,111('final volume of emulsion desired must be about halved inlilt' making.

Formula [or I liier of emulsion (roOD cc)

A. Water · ············ 350 ccAmmonium bromide............ 32.5 gPotassium iodide . . . . . . . . . . . . . . . . . . . . . . . 0·5 gGelatin 10·5 g

B. Silver nitrate 25 gDistilled water 5 ccAmmonia (cone.) a sufficient quantity to red is olve

the precipitate of silver hydroxide first formed(this will require about 25 cc of ammonia of0.920 s.g.)

C. Silver nitrate '. . . . . . . . . . . . . . .. 25 gDistilled water IIO cc

l h-rt- we see that about 500 cc of water is divided between the.rll s and silvers in the rough proportion of 35 to IS. The bro-

IIl1dt'is used in about 8.5 per cent excess of the combining quan-I11 . The iodide is present in the proportion of one per centIII Ihe weight of silver nitrate. The gelatin in this case is three111'1('('nl of the water in which the salts are dissolved. The silver1111I:tie' is divided into two equal parts, one of which is highly1llll('('nlraled and redissolved with ammonia; the second half is

66 PHOTOGRAPHIC EMULSION TECHN IQUE

used plain, and is dissolved in just over four times its weightof water. Plain silvers should not as a general rule be used ina higher concentration than one to four of water.

The' excess of bromide used over the weight needed to com-bine with a given quantity of silver nitrate vanes over widelimits. Wall 2 quotes cases where the excess varies from 1.9parts per 100 of silver nitrate (Burton) to 92.4 parts per 100(Eder ). Eder is also quoted as using in different emulsions 8.4,67-4, and 75-4 parts respectively per 100 above the combiningquantities of Ag 03, indicating at least that there is a verygreat tolerance. He states that the explanation usually acceptedfor the favorable action of the excess bromide is that the finer.particles of silver halide are dissolved by the exce s and in thecourse of ripening are deposited on the larger grains (er Ost-wald ripening), thus giving increasing sensitivity. It will befound, however, on studying the earlier literature that a largerexcess of bromide was used in cases where all or a large pro-portion of silver nitrate was redissolved, and this indeed appearsto be necessary in order to prevent fog. Only a very small ex-cess is actually needed where the silver nitrate is divided intothree or even four parts and added to the salt piecemeal, withvery little ammonia, allowing fairly long intervals between ad-ditions; or where the iodide is added separately after the firstemulsification with silver and bromide only, with or without am-monia, when it appears to act as a peptizer.

The three solutions in the above formula are heated, A andC to IIOo F. (43° C.), and B to 70° F. (2Io C.). B is pouredin a steady stream into the crock containing the salts A, stirringfairly briskly, the precipitation taking about five seconds. Cis then at once poured in, this time through a funnel with athree or four millimeter outlet, stirring briskly. After ripeningfor fifteen minutes, without stirring, or at least giving only an oc-casional gentle stir, the bulk gelatin is added, dry, about 70 g

NEGATIVE ElVIULSIONS

being used, which will bring the concentration up to approx-imately eight per cent of the final bulk of 1000 cc. A little gel-atin, together with a small amount of the silver halides, is in-evitably lost in washing. Fifteen minutes should be allowedfor the gelatin to dissolve completely, even if powder be used,the crock being gently stirred the whole time. The tempera-ture must not be allowed to rise above 105° F. (40.5 ° C.) duringIhi operation, but if the same emulsion be made on several oc-t asions, the same temperature conditions should be repeated asfar as is possible. When the gelatin is dissolved, the emulsionis set off by placing the crock in ice water, and giving the con-I('nts an occasional stir until gelling takes place.

The above way is a convenient one for expressing the formula,but only one of 'many. Another, frequently met with in Eu-ropean literature, would be as follows:

(n) 180 cc gelatin solution 50/0(/I) 162.5 cc ammonium bromide solution 20%(r) 5 cc potassium iodide solution 100/0

(d) 2$ cc silver nitrate solution I: I

({') 100 cc silver nitrate solution 25%(f) 70 g gelatin, dry

l'his is very approximately the formula already given. It is ofrourse a great convenience to use reacting substances in theIorm of solutions, especially silver nitrate. In some factoriesIlu-se are made up in the form of multi-normal solutions ofwhich equal volumes are equivalents. In several of the for-111111asgiven in this book, where highly concentrated silvers arei-mployed, it will be found more practical to weigh the silveruurate out.

1\ further method of setting out a formula is used by Carroll011111Hubbard 3 in the case of an " all-ammonia" emulsion, and\\1' find that they have expressed it in more academic fashion aslollow :


VVater D}l 250Ag OR (0·353 mol) g 60NH

40H cone. (0.71 mol)

to redissolve

VVater .NH4Br (0-409 mol) .KI (0.0035 mol) .Gelatin .

ml 500g 40

g 0·58g 30

Excess bromide is seventeen per cent. The halide-gelatin solu-tion was kept at 45° C. -+- 0.5° during mixing, which tookfour and one-half minutes, and during ripening. After ripen-ing for twenty-five minutes, 70 g of gelatin previously swelledin cold water was added, taking seven to ten minutes, and theemulsion was chilled. (The mol figures given are obtained bydividing the molecular weight of the substance into the weightused.)

Another distinctive type of emulsion is that recommended byHeyne 4 for negative motion-picture film. Three solutions areprepared as follows:

A. Gelatin .VVater ...............•.................Potassium bromide .Potassium iodide .

B. Silver nitrate .Distilled water .Ammonia (s.g. 0.910) about

C. Gelatin .Distilled water .

250 g3500 cc400 g

10 g

500 g1500 cc475 cc

500 g2500 cc

The gelatins in solution A and C are allowed to swell in thewater. The salts are then added to A and both solutions areraised to 122° F. (50° C.). The silver nitrate i dissolved inwater, and ammonia added as usual, sufficient to redissolvethe silver hydroxide. This is used at room temperature and isadded slowly to the crock with good stirring.

The emulsion is maintained at 112° - 122° F. (45°-50° C.) for thirty minutes, when the gelatin solution C is addedto it at the same temperature. The emulsion is then digested

EGA TIVE EMULSIO S

for a further fifteen minutes, and' is set off in ice water. It isstated to attain its highest sensitivity during the digestion afterwashing or finishing. A temperature of 112° - 132° F. (45°- 55° C.) is recommended for the finishing. In this process asmall trace of ammonia may be added. (This addition is notrecommended by the author unless a second washing be given,when the bulks will require modification throughout, or the pHbe lowered to just above normal by suitable treatment with thefinals.) During the final digestion, small hand-tests ar,e made atshort intervals, and when the maximum useful speed and gammahave been obtained, the usual additions of chrome alum, bro-mide, spirit, etc., are made. In the case of film coating, one toone and one-half per cent of glycerin may be needed.

Let us assume that the emulsion has been given its correctripening time, and that the bulk gelatin has been added. It isimportant that this be very thoroughly dissolved. Powderedgelatin should be " poured" slowly into the emulsion with goodsi irring ; this will avoid its annoying tendency to form into glueylumps which are troublesome to dissolve. In making small lotsof emulsion it will be found a convenience to cut up sheet gel-:t Iin beforehand into small pieces with a clean pair of scissors.The gelatin melted, the crock or jar is stood in ice water andIhe emulsion allowed to set. It should always be given an oc-t asional stir until it begins to gel, especially in the case of largeha tches. These, if of any fast variety, are best distributed over:t number of small 'one and one-half or two-gallon cold crocksto expedite setting. In the case of emulsions where the wholeCIf the silver has been treated with ammonia, washing is usu-.rlly given within from three to five hours, otherwise ripeninggm's on in the cold, and fog may result. With emulsions whereItoss than fifty per cent of the silver has been converted, it isc ustomary to leave the emulsion in the cold room until next day.111 fa tory practice it is of course important to have some system

PHOTOGRAPHIC EMULSION TECHNIQUE

by which the time between emulsifying and washing is keptmore or less con tant for each type of emulsion. Thus if all-ammonia and part-ammonia types are being made, the all-ammonias can be mixed in the morning and washed in the after-noon, while the others are mixed in the afternoon and washedthe following morning. It will be found a convenience to takethe set emulsion from cold storage and leave it at room tempera-ture for two or three hours before washing, otherwise it may I

be unmanageably stiff for shredding. This applies more par-ticularly to small lots or types containing a high concentrationof gelatin.

Small lots of emulsion can be broken up with the hand, and·pieces of the size of an orange wrapped in a piece of coarse net-ting (about one-eighth-inch mesh) and squeezed through thematerial. The netting is such as is used for embroidery work,and must be very thoroughly washed before use, as the sizingwill otherwise cause trouble in the way of fogging or desensitiza-tion. Various devices used in cooking which can be bought atthe hardware stores or department stores have been recom-mended from time to time for breaking up the emulsion, but thedanger here is that the zinc from any galvanized parts will fogit, and although a textile material of sufficiently large mesh is

.more trouble to use, it is definitely recommended. The jellycomes through the netting in the form of broken shreds aninch or two long, and if the emulsion be held under the waterwhile squeezing it through, the bits or noodles will keep nicelyseparated. They can then be transferred to the washing jar.A piece of muslin or cheesecloth (also previously washed out)is tied over a jar with a bottom outlet as shown in Fig. 22, theshreds put into the bag formed by the material, and the jarfilled with water.

At the end of five minutes the plug is removed and the waterrun out, and three minutes is allowed for the shreds to drain.

NEGATIVE EMULSIO S 71

The draining may be made more thorough by teasing the shredswith a glass rod or spoon. The washing procedure is then re-peated, and after about eight such changes a te t of the washwater with Nessler's reagent will usually reveal that the am-

----------------

------------- - - -------------

FIG. 22

monia has been eliminated. If not, one or two further changesshould be given. The same procedure applies to works-scalewashing, but here it is most important to insure that the swell-ing of the noodles has not been too much, so that on remeltingand addition of the finals, the bulk is too large. The best prac-tice is to use tared crocks or nickel or stainless steel pans, andto weigh the emulsions rather than measure them with a stickor any form of graduation.

Considerable difference of opinion exists as to the best lengthof time for washing. Bogue found 5 that the swelling and vis-cosity of a gelatin, which are their minimum at a pH of 4·7, in-

PHOTOGRAPHIC E1VIULSION TECHNIQUE

crease regularly with a.rise in pH up to about 8.5, but that abovethat value they decline slightly, due to an increasing solubility.As washing proceeds, and the pH becomes lowered, the swellingis probably reduced, and it is advisable to get ·'d of the greatbulk of ammonia in the first stage of washing by quickerchanges, making the later changes of longer duration. It wascustomary in the early days of emulsion-making to give severalhours; more recent literature indicates two and one-half to fourhours. If the noodles be kept thoroughly separated duringwashing, and are carefully drained in the intervals between thewater changes, sixty to eighty minutes should suffice, except incases of highly ammoniacal or of color-sensitive emulsionswhere any excess of bromide may retard sensitization. The finalwash water should give no 'reaction with silver nitrate as com-pared with a control; the Nessler test will reveal the presenceof residual ammonia. Here it may be stated that distilled waterfor washing has not proved satisfactory in general practice. Inmost cases poorer speed and slight fog seem to result from itsuse. That the character of an emulsion can, however, depend ina remarkable degree upon the characteristics of the local waterused for washing is well proven. Wash water has in certaincases been prepared by adding to distilled water the ingredientsfound by analysis in a natural water which has proven satis-factory. But as a general rule the town supply, or suppliesfrom artesian wells, will be found perfectly good. The presenceof mineral salts affects to some extent the swelling of gelatin.Patents have been granted for the buffering of emulsions, main-taining the hydrogen ion concentration within a range of pH 5to 10 by various additions, as disclosed in recent Kodak pat-ents." The pH of the washed negative emulsion may vary be-tween 7 and 8.5, though acid emulsions will of course have alower pH.

A method adopted by Carroll and Hubbard 7 for shredding

TEGATIVE EMULSIO S 73mall lots was to set the ripened emulsion by pouring it in a

layer of one cm to two cm thickness into a clean enameled traylloated on water at S° to 8° c., and leaving it overnight. Thejr-lly was then pressed through a sieve built up from sharpened1III'Ialstrips set on edge, which cut the emulsion into clean shreds11\'1'lo six mm in cross section. The shreds were placed in silver-pl.ltcd cans with twenty-per-inch mesh screens of pure nickel at1111'bottom. The washing was stated to be very thorough, lastingI I() eight hours, with frequent hand stirring of the shreds. In

1111'ab ence of ammonia in the emulsion, the pH after washing\\.IS 7 -+- 0.5; if made with ammoniacal silver oxide the pHIlIi~ht be as high as 8.5.

'I'hc emulsion shreds having now been washed so that they1IIIISisl of merely insoluble silver halides in gelatin, they arelI'IIII'IICdin a water bath, and any final additions made. In re-1111'11ing, a good bulk of really hot water is desirable, so that the1I111I(1I('smelt as quickly as possible. The temperature must111'\I'l'lheless be kept ~vithin a degree or two of that at which11",dil-(cslion is to be carried out, especially in the case of fastI 1I11I1~i()ns.The zemelting is an essential part of the process,d m.ik ing, and it is highly important in routine work to control11" u-mperature employed, and the overall time taken. As al-II .u l indicated, an alternative to remelting a number of small11111IlI's is to blend the washings from a number of crocks and111111I1 in bulk.

I Ill' linal digestion or finishing is carried out in thermostati-I "" I ontrolled water bath such as have been described in theI" I lOllS hapter. Although there is actually no specific con-11111111'link between final speed and

Time X Temperature

r r] ""11 illl-(,it is certain that above 120° F. (490 C.) some crit-h d t.1~(' is reached where sensitivity very rapidly increases.


The temperature of the finishing is another matter on whichsome divergence of opinion exists, but this is rather because thecritical point depends so much on the particular formula. Onemight hazard the generalization that fast negatjve emulsionsmade with ammonia are best digested at a temperature notabove I25° F. (520 C.), while slow emul ions, and particularlythose prepared with acid, are best treated at I600 F. (7IO C.)or somewhat under. The finishing time may be as short as halfan hour in the former type, or it may run into three or fourhours in the latter. This can only be a very rough comparison,and the extent to which the time may be shortened by raisingthe temperature is a matter that must be found entirely by ex-'periment for each individual emulsion formula. Probably lesssedimentation takes place at the higher temperature and lesstime, in spite of a lowered viscosity of the gelatin.

An interesting example of the effect of the final digestion uponspeed is shown by the curves given in Fig. 23. The fact thatwashing was in this case clone by centrifuge does not interfere.Four lots of one particular emulsion, ripened initially for (c)5 minutes, (b) 30 minutes, (c) 60 minutes, and (d) I20 min-utes, were digested after washing for periods up to two hours.It is seen that while in all cases there is a tendency for the speedto rise to a maximum and then to fall off, the highest speed isobtained when the ripening has been longest. With increase inripening time, however, the regression of the speed after themaximum has been reached is more pronounced.

In discussing the effect of grain size on the finishing of emul-sions, Trivelli and mith S state that before starting the finish-ing operation, which was done at 600 C. (I400 F.), the emul-sions, with the addition of gelatin and water, were brought upto a fixed weight and to the same temperature, 400 C. (I04 0 F.).(( In this condition," they say, (( the emulsion is in its loweststate of sensitivity." They further state that the increase in

EGA TIVE EMULSIO S 75300~------------r-------------r-------'

IIIWo,If)

200~------~L-~--------~~t-~~-i

IOO~---J~--~--~------------r---~r;

/ 2HOURS DIGESTION

)ill:. 23. SPEED OF CENTIFUGED AMMONIA-PROCESS EMULSIONS AThit I< TIMES EACH OF RIPENING ANDDIGESTION. Ripening time (reading

Ill' l rorn bottom) curve (a), approximately 5 minutes; curve (b), 0·5huur, curve (c), I hour; curve (d), 2 hours. (Carrell and Hubbard)

11I'('d'an be expressed by the ratio between the Hand D speedloIlI.lilled after digestion at 600 C. and the speed measured atI" t ', imilarly the increase in gamma can be expressed byIltl nuio between the gamma obtained after digestion and theF.llIlIllil measured at 400 C. before digestion.

I1 IlIay be aid, by and large, that the time required for finish-"11' III f't nal digestion must be found by trial and error for everylmlrvidual emulsion. One method suggested is to make ((wet


tests" at frequent periods during the cooking. Suppose, forexample, that we are finishing a washed emulsion by digestionin the water bath at 600 C. Every five or ten minutes, a fewcubic centimeters can be pipetted from the crock and coatedupon a small sheet of glass and set on the ice-cold levelling table.As soon as it is thoroughly set, it is exposed behind a step-wedge(p. 210) for a fixed time and developed for a fixed time at afixed temperature. The necessary apparatus will of course beavailable. If the step-wedge be mounted in a deep printingframe, with a thin fillet of wood or aluminum around the edgeabout a quarter-inch wide, the wet plate can be carefully laiddown on the fillet frame and the exposure made without soilingthe wedge. Such a test frame should be part of the equipmentin every emulsion laboratory. A series of wet tests of this typegives a good visual indication as to when the climax has beenreached, by showing more and more steps as the cooking time isextended. After x minutes, when the maximum useful speed hasbeen attained, either fog will step in or the speed will actuallydiminish, as seen in the series of curves shown in Fig. 23. Onfuture makings, check wet-tests of the emulsion at short inter-vals when approaching the optimum time of digestion will con-trol the resul t, though they should not normally be necessary inworks practice.

The reader is advised, when making his first negative emul-sions, to give the final cooking at 1250 to 1300 F., and to coatplates at intervals of ten minutes or so, using a measured amountfiltered through one thickness of muslin in a beaker; and to drythese plates in the cupboard, and make strict comparative testsas to speed, gamma, and fog, on the lines discussed in ChapterXII. A suitable coating weight must be made, as otherwise themeasure of the fog will be badly out. These tests will furnishinformation for the guidance of future makings, and provide thebest means for investigating any formula, or for checking up a

NEGA TIVE EMULSIONS 77

IIC'Wsample of gelatin with a known formula. In such tests it isinadvisable to add any finals such as free bromide, alum, pre-ervative or spirit.

lt has already been stated that an emulsion comprising grainsIIf a large number of diameters gives the best gradation. It hashcen frequently recommended that to obtain this quality a mix-IIIre of coarse-grained and fine-grained emulsions should be11eel. If, for example, a very rapid emulsion be made in which;1long ripening has brought about the growth of a high propor-1ion of coarse grains having high sensitivity, the admixture of.1slower but more contrasty emulsion having finer grain wouldlu-lp to build up highlight density without affecting the shad-IIWSappreciably. Jahr 0 states that highly sensitive but flatr-mulsions can be mixed in various proportions with contrastyr-mulsions of medium or even low sensitivity. The additionId 01very fine-grain emulsion of high gamma infinity not onlyIwlps to increase highlight density, but adds opacity to themlxr-d emulsion and makes possible a more economical coating\I I·ighl. On the other hand, it must be borne in mind that whenfilling emulsions having a different iodide content, the power-1111affinity of iodine for silver may cause some reaction of oneI rnulsion on the other; it is not therefore entirely possible toI'It.diel from the characteristic curves of two emulsions that are11\1I'cI, exactly what the characteristic curve of the mixture1\ III he.

I1 is a more reasonable procedure to provide for the precipita-111111of grains of the various sizes required in one single emul-11111,and this can be done by bearing in mind the point which

It,I\'I' already been discussed in connection with emulsification.\\ I' I·now, for example, that a concentrated solution of silverIIlll.tll', re-dissolved and" flopped" wholesale into the crock,

III rivr a more or less homogeneous mixing of equal-sized grainsI1 IlIltlring high contrast with heavy maximum blacks and low

PHOTOGRAPHIC EMULSION TECHNIQUE

latitude. We know also, that inasmuch as grain size increaseswith ripening time, the silver could be added in small quantities,spread aver a period, so that when the ripening was cut shortby the addition of the bulk gelatin, the various Jt,.recipitates ofsilver halide would have grown ta respectively different sizes.The grain-size frequency curve of an emulsion is an importantguide to its characteristics. Owing further to Ostwald ripening,prolonged heating tends to give a maximum of coarse grains,these having grown at the expense of the smaller ones. Hence,by dividing the silver into two, three, or even more parts, andadding them one at a time, it is possible to make an almost un-limited variety of precipitable mixtures providing equally an .unlimited variety of characteristics in the finished emulsions.

A further alternative is to 'add the first silver in the form ofa plain, moderately dilute solution (say 1:4 concentration) andafter an interval of a minute or two to add the iodide. The re-mainder of the silver is then added. This tends to increase thetime required for ripening, and there is some evidence thatthe effect of the iodide so added is that of a peptizer, dispersingthe initial silver halide precipitate. A variation, therefore, ofany formula in which the silver is added in stage, is to add thefirst silver plain (though some ammonia hould be present in thesalts solution), then to add the iodide, preferably with a smallquantity of ammonia, and finally to add the remainder of thesilver in one or more additions. A small quantity of ammoniashould always be present in either the salts or the fir t silversolutions, otherwise clots or agglomerates of precipitates maybe formed, which will cause black spots on development with-out exposure, owing to insufficient colloid protection. Silverbromide without a clothing of gelatin will be spontaneously re-duced to black metallic silver an treatment with an alkalinedeveloping solution.

Although, with a proper choice of gelatin and a goad formula,

_JEGA TIVE EMULSION S 79

maximum speed should be obtainable without fog, several sub-stances have a useful effect in keeping away fog, which reallymeans in destroying the developability of over-ripened or over-digested grains. Thus in his well-known book, Ausfuhl'lichesIl andbucb del' Photographie, Dr. J. M. Eder gives as substances(hat destroy latent chemical fog, tincture of iodine, bromine,chlorine, and minute quantities of a dichromate acidulated withmineral acids .. Against this, sensitivity is considerably loweredIlY additions to an emulsion of bromine or iodine, or the metallicpcrchlorides. The most useful anti-fogging addition is prob-nbly a trace of hydrobromic acid, which is frequently made toan orthochromatic emulsion which requires to be fog-free butneed not yield any shadow detail, such as far example the so-railed document type of emulsion. A few cubic centimeters ofa I: 100.0 solution of hydrobromic acid may be added to oneliter of the emulsion, but its effect should be carefully tested()\l peed and on the foot of the characteristic curve before suchlr atmerit is adopted. A trace of cupric chloride is sometimesused in collodio-bromide emulsions for the same purpose. Morerecently, anti-fogging substances have been found for use. inmaking extremely high speed emulsions, and thereby makingpossible some of the newer family of films which have so changed(he complexion of modern instantaneous photography; refer-cnce to these will be found in Chapter XIV, to which the readeris referred.

The phenomenon of halation, as such, needs little explanationhere. 'While irradiation or reflection of light from the emulsionparticles within the film can cause a spreading of the im~ge,(he hief trouble is met with in glass plates, where rays of light\\1(' Una the film obliquely from the len strike the far side ofthe alas support, and are reflected back therefrom, reachingt he emulsion again at an appreciable distance from the originalpni nt of the image. The application to the back of the support

PHOTOGRAPHIC EMULSIO J TECH IQUE80

of an absorbent layer having a refractive index as nearly as pos-sible that of glass, was originally suggested by M. Carey Lea.A later method was to interpose a light-absorbent substratumbetween the emulsion and the glass or film base This sub-stratum could be colored with a dye that could be destroyedby an acid fixing bath, or discharged by the use of anyoneof several other similar treatments which do not at all affectthe image.

The idea of employing a light-absorbing film of gelatin con-taining, for example, curcuma or fuschin, was suggested in r878.An acid dye would be used, being more readily washed out ofthe gelatin. As a substitute for a dye, a 0.6 per cent gelatinoussolution of potassium permanganate has been suggested byOakley.>? The permanganate is reduced during developmentto more or less colloidal manganese hydroxide which is readilysoluble in the average acid fixing bath, or it may be removedby the use of a two per cent solution of sodium bisulphite. Var-ious types of anti-halation treatments are now applied to highspeed panchromatic films, for particulars of which the readeris referred to the patent literature. In the case of films theEastman Kodak Company has used a magenta dye in the 'non-curling layer applied to the back of orthochromatic film, and agreen layer with certain panchromatic materials. The chemicalcomposition in each case i such that they bleach out completelyin a" properly compounded developer and fixing bath." In thecase of thirty-five millimeter film, a blue-gray dye is actually in-corporated in the base. Light transmitted by the emulsion mustthus pass twice through the dye in order to get back to the emul-sion and cause halation." The blue-gray dye is therefore twiceas effective as it would at first appear. It does not bleach outin the processing operation, but its presence has no ill effect onthe printing quality.

A further type of fast emulsion which may be dealt with in

JEGA TIVE EMULSION S 81

Ih is chapter is the reversal film used with many kinds of naturalrolor material. The Autochrome plate, introduced in 1905 byIhe' Lumiere Company, gave a positive image in natural colorshy means of such an emulsion. The color matrix consists ofsIarch grains colored blue-violet, green, and orange, in prede-Iermined proportions, and on top of this is coated a very thinlayer of reversal type emulsion. The plate (or film) is exposedIhrough the back, hence the silver halide grains must be ap-PI"(' iably smaller than the elements of the color matrix. TheIII'st development gives a negative image, which is convertedhy reversal into a positive. The reduced silver is soluble in an,it id olution of potassium permanganate or in chromic acid,while the unexposed and therefore unreduced silver bromide isnot, Hence after" bleaching," the original negative image willdisappear, leaving transparent gelatin, while the unexpo ed sil-ver bromide remains. If, therefore, this be now exposed and.h-vcloped, it will be in turn reduced and will obviously givehlnrk where there was white in the negative image. The nega-Iive "blacks" being dissolved out' and showing white whenviewed by transmitted white light, a positive of the original sub-Jt'cI i obtained. In the old Agfa color process (The Net» AgfaIiIIII i a triple-coated continuous image type, and does notdl'Il('nd upon a matrix. See p. r 22) the color matrix, and the111:11rix and the" reseau " used in Dufaycolor film, are sim-1i:1I'lycoated with these reversing emulsions. Reversal of thisIVpt' must not be confounded here with the reversal that occursI11ally negative emulsion on gross overexposure, represented inIlu- characteristic curve by the turn of the shoulder where den-uy decreases after a certain maximum of progressive exposure.\ It exposure of something like one thousand times normal is111'I'clrc1to give a reversed image by straight exposure and de-'l'lopment, but such a method is entirely u eless for color-screen

"Inll's and films.

82 PHOTOGRAPHIC EMULSIO TECH TIQUE

Owing to the fact that the colored elements of a good matrixfor a direct natural-color process must be exceedingly small,the need for an extremely fine-grained emulsion is self-evident;yet it must be fast enough to make snapshot exposwes practical.The potato starch grains of the Autochrome matrix, separatedout by levigation, vary in diameter between 8 J-Land 20 ~l, aboutfour million going to the square inch. The Dufaycolor matrixor reseau has a geometrical formation of alternate lines 500 tothe inch, which are dyed red, and lines made up of alternategreen and blue rectangles about one-two hundred and fifty thou-sandths of an inch each in area. The function of the reducedsilver grains in any such process is to block out any unwantedcolor element when the whole image is viewed by transmittedlight, or projected. Thus in the case of a green leaf, the imagemust be transparent only over the green elements, and the blueand red elements must be blocked out by opaque silver deposits.To get this effect as perfectly as possible, the requisite numberof grains must be packed into the unwanted area. Yet if thefilm be too thick or too deep, we may get scatter effects as wellas parallax. The desideratum is therefore to have a fast, veryfinely grained emulsion with such density-giving powers thatthe thinnest possible coating may be adequate, giving an imageas nearly as possible in optical contact with the calor matrix. Acoating weight determination (p. 231) of an average color-screen film will indicate something between 35 and 65 mg ofsilver halide per square decimeter - about half that of an aver-age negative film. Even then a developer containing a silverbromide solvent such as a thiocyanate must be used in the firstdeveloper in order to get an image of sufficient clarity.

It is by no means certain that we have heard the last of color-matrix types of natural color film. The immense amount ofwork done by Powrie and others indicates possible success ofa highly commercial character, in spite of the success of triple-

I EGATIVE EMULSIONS

.oated subtractive films. A very full description of the manylines of attack on the calor matrix problem is given in E. J.Wall's" History of Color Photography." 12 Where an emul-sion is required for experimental work of this kind, the reader isadvised to confine his first attempts to a speed of 200 to 250

Hand D. Reference to the hypersensitized emulsions of veryhigh speed will be found in Chapter XIV. An emulsion of thespeed named can be super- or hypersensitized after coating bybathing the dry plate or film in a solution of ammonia and alco-hol in the proportions:

Distilled water .Stronger ammonia water .Alcohol .

1000 CC

3 cc250 cc

A 0.2 per cent solution of triethanolamine or borax will act sim-ilarly, or even soaking in distilled water for three minutes anddrying in a warm current of air.. Bathed material may be oneand one-half to two and one-half times the speed of the originalemulsion, but it keeps in good condition for only three or fourdays, so it must be prepared only slightly in advance of thetime when it-will actually be needed.

A method of combining initial high speed with fine grain inthe reversed image was referred to by the author in 1932.'3 ByII ing the most sensitive, larger, grains for the exposure of the11 gative image, and having in the emulsion a sufficiency of smalldiameter and relatively insensitive grains left over, the latterwill, on adequate exposure and development after bleaching outof the initial image, give a reversed image of good resolvingpower and adequate blocking-out power. Such an emulsion canbe made by dividing the silver into two parts, one being re-dissolved and the emulsion ripened, after its addition, in con-siderable excess of alkaline halide, the other silver being addedplain with no ripening time allowed:

PHOTOGRAPHIC EMULSIOl TECHNIQUE

A. Water : .Ammonium bromide .Ammonium iodide .Soft gelatin .

B. Silver nitrate .Water .Ammonia (s.g. 0.910) .

C. Silver nitrate .Water to make .

2500 cc380 g

5 g80 g

300 g100 CC

q.s. to redissol ve

300 g1200 CC

Solution B is added at 850 F. to the crock containing A, whichshould be at roo? F. After fifteen minutes initial ripening, solu-tion C is added through a three to four miIIimeter nozzle withbrisk stirring. The bulk gelatin of 720 g is immediately afteradded dry, and the whole stirred for fifteen minutes, bringing thetemperature up to roSo F. When dissolved, it is set in ice water.After very thorough washing, it is digested for an hour atr 250 F. and the color-sensitizing dyes are then added in theusual way. It is finally made up to ten liters with the additionsbelow:

Spirit (less the amount already added in the .form of dye solutions) 500 cc

5 per cent chrome alum solution. . .. . 100 cc10 per cent phenol solution 100 ccI per cent ammonium bromide solution 50 cc

The emulsion is richer in silver than is usual with negativeemulsions which are to be used for ordinary photography, butin order to keep the film compact and very thin, only about halfthe usual coating volume is applied. The gelatin may advan-tageously be still further reduced if suitable coating facilitiesexist.

Reversal emulsions are steadily becoming of greater impor-tance. Quite apart from color work, practically all sub-standard motion-picture work is of the reversal type, and im-

EGATIVE EMULSIONS 85

mense quantities of 8 mm, 9.5 mm and r6 mm black-and-whitereversal films are manufactured. Reversal emulsions are alsonecessary for triple-layer sub tractive color films, and their ap-plication is being extended to other purposes where the produc-tion of a direct positive image is of advantage. In sub-standardmotion-picture work, it is to be remembered that great enlarg-rnent of image.. is involved in projection, so that extremely finegrain is of the utmost importance, always provided that it canbe coupled with adequate speed.

Chapter Rejerences

1. L. P. Clcrc, Photography, Theory and Practice, p. 134.2. E. J. Wall, Photographic Emulsions, p. 33..I. Carroll and Hubbard, Bur. of Stands. Paper No. 340..i. W. Heyne, Handbuch del' Wiss. und Angew. Phot., p. 268.S. R. H. Bogue, Chemistry and Technology of Gelatin, p. 184.(l. B. P. 492, 162 and 492, 257, etc.7. Carroll and Hubbard, Bur. oi Stands. 1. oi Res., 7, 193I.11. TrivelJi and Smith, Comm. No. 704, Kodak Res. Lab.'J. R. j ahr, Handbuch del' Wiss. und Angew. Phot., p. 229.

'0. C. F. Oakley, E. P., 2986, 1895., i . "Kodak Films," p. 16, I940., . American Photographic Publishing Co., Boston, Mass..I. T. T. Baker, Photo Journ., 72, Mar. 1932.

CHAPTER VSLOW EMULSIO S

Sensitive Material for Copying, Commercial Work and Transparencies-Methods of Producing High Resolution and Fine Grain - Chloro-bromideEmulsions - Chloride Emulsions - Mixed-jet Emulsification - Devel-opment of Warm Tones.

IT has been seen in a previous :c~apter that a f~st neg~tiveemulsion can be made by " boiling " a suspension of silver

bromo-iodide in gelatin until the grains are sufficiently ripenedand digested. In this type 'of emulsion it is probable that sen-sitization takes place as much in the ripening as in the finishingstage. It has been seen also that by the use of ammonia thetemperature and time needed for arriving at a similar speedand character may be greatly modified. It is doubtful whetherthe extreme speeds attained in recent years can be produced bythe boiled method.

We come now to the problem of making comparatively slowemulsions, of 25 to ISO Hand D. Such emulsions play a veryimportant part in industry. They are indispensable for copy-ing work, for recording purposes, for the making of transpar-"encies, and for various branches of photo-mechanical reproduc-tion. The coating of positive emulsions on film stock of courseruns into enormous figures in the motion-picture industry. Somesix thousand million feet of thirty-five millimeter film were usedin I933.1

In what way, may we ask, do these slow emulsions differ fromrapid ones? The answer is that a slow emulsion is made withsubstantially finer grain, which is not allowed to grow to anyextent in ripening. This is controlled by the concentrations ofthe solutions employed in precipitation, by temperature, and

SLOW EMULSIONS

largely by pH. A slow emulsion may easily be made to developiI gamma of 3.0 or 4.0 and may indeed be required to do so, butit has a fine grain and a high resolution coefficient. The fasti-mulsion will probably develop a gamma of no higher than 1.0

t () 1.8. Apart from improved resolution, irradiation is reduced;

3r-----------------~------__,

.LOG 'EXPO,sVRt:

FIG. 24

III lit her words spreading of fine line images is minimized, and1111'1 t' is a less tendency to halation.

l'he production of slow, fine-grained emulsions must be di-Idl'd inla two classes, (a) the photo-mechanical type, where

III ut-eel either blacks or whites with no intermediate tones, andI high inertia is of no consequence but rather an advantage,IlId (h) brilliant and contrasty emulsions where the densityI III 't' is long and latitude is of a high order, but with whichIlIl'h maximum blacks can nevertheless be obtained where de-III-d, The type having a high inertia, and giving intense blacks


on heavy exposures butinsignificant densities on low exposures,is seen in characteristic form in Fig. 24, as compared with atransparency type of emulsion having low fog, fairly high max-imum density, but a long even scale of gradation Both typesshould be made with a high concentration of silver in order thatthe actual film coating may be as thin as possible.

The first type is made most easily by precipitating the silverhalides as far as possible in a uniform grain size. In practice,it is found best to pour concentrated silver nitrate rapidly intothe salts solution and to add the bulk gelatin immediately afterprecipitation so as to inhibit ripening, which is synonymous withgrain growth, and also leads to the formation of grains of dif- .ferent diameters, which in turn gives a milder slope to the char-acteristic curve. Against this, we need brilliant images, lowdensities and absolute freedom from fog, good gradation andan attainable gamma of 2.0 to 3.0 to provide a first-class trans-parency emulsion. Our knowledge of the laws of precipitationindicates that pouring the silver in a fine stream with rapid stir-ring and the use of more viscous solutions, should give thephysical result. The production of fine grain needed for photo-gravure or diapositive work demands not only a more viscoussolution for the salts, but reduced silver bromide solvents suchas excess alkali halides and ammonia in the ripening. The ad-dition of silver at a moderate concentration or only partiallyredissolved through a nozzle at a slower rate, with mechanicalstirring of the crock, or the pouring of the silver and salts solu-tions simultaneously through two jets or nozzles into a solutionof plain gelatin, gives the" process" character with the longerscale of gradation. Mixed jet emulsions of this type with thewhole of the silver redissolved will yield very great contrast,especially if the bulk gelatin be added immediately after precipi-tation. In very slow emulsions, such as chloro-bromide andchloride types, the substitution of hydrochloric or citric acid

SLO~T EMULSIO S

for ammonia is customary, the citric acid often being dividedlu-tween the salts and the silver.

The following characteristics may be taken as representative11r the slower types:

Emulsionl'hotogra vure .1'1 ()CCSS •.......•..•••...•.....•••••

l.nntcm .('hloro-bromide .Chloride .

Speed,H&D

Densityrange

Gammainfinity

1.860-IOO 1.52$-60 2-3·56-10 2·51-2 2·5

0.I-1 4

3-53

4

It has already been pointed out that the characteristic curvej steepened and latitude shortened by having more or less homo-I'C'I1COUSgrains of similar diameter. This condition is obtainedill the formula given below by adding a concentrated redissolvedilver at a low temperature to the salts, adding the gelatin at

1111('('and setting as quickly as possible. Alternatively the saltsdltel silver can each be dissolved in the same bulk of water andPIIlII'cd into the crock simultaneously with the apparatus de-• C I ibcd on p. reo. The gelatin solution may be made alkalinewit h ammonia, or acid with a mineral acid to neutralize the am-1I11111iaset free by reaction of the silver nitrate with the alkalineh.i lirles. This helps control the tendency for rise in speed. /

Ilparing in mind that a process emulsion when coated re-I(1111PS about sixty to eighty milligrams of silver halide perqunre c1ecimeter, and that the gelatin-silver ratio should be

Il'pt low, we may formulate the emulsion somewhat as follows:

A. Ammonium bromide .Ammonium iodide .Gelatin .Water .

B, ilver nitrate .Water .

tronger ammonia water .

230 g5 g

200 g

4000 cc

350 g50 cc

q .s. to redissolve

90 PHOTOGRAPHIC ElVIULSIO TECHNIQUE

The gelatin is soaked for thirty minutes in the salts solutionA, which is then heated and the gelatin dissolved. It is thencooled to 950 F. (350 C.). Solution B is used at room tempera-ture, 700 F. (21

0 C.), and is poured on quickly out of a stone-ware jug, taking two or three seconds only. The crock shouldbe vigorously stirred, if possible by mechanical means. Irn-medialely after emulsification, 550 g of powder or broken gel-atin is added, the jar being stood in water at about 1600 F.(700 C.) to facilitate quick melting, but the temperature of theemulsion itself must not be allowed to rise above 1050 F.(40.50 C.). The gelatin will take ten to fifteen minutes to dis-solve completely. The emulsion should then be transferred to .an ice-cold crock or a number of smaller ones and stood in icewater, stirring until gelling begins. Not more than three tofour hour should be allowed between setting and washing. The \final wash water should give a clean N essler test. When washed,250 cc of spirit is poured over the shreds, which are then re-melted in the usual way, and the emulsion is digested for abouttwo hours at 1250 F. (520 C.). One or two cubic centimetersof ten per cent glacial acetic acid solution is added and wellstirred in, sufficient to bring the pH to just below 7. The re-mainder of the alcohol, 250 cc, is then added, together with100 cc of chrome alum solution and 50 cc of ten per cent potas-sium bromide solution. The emulsion is next filtered and setoff, being remelted next day or when required for use. Suchan emulsion will keep in the cold room for several days, butshould always be checked for fog by a wet test before beingsent to the coating room. The total bulk of the above emulsionshould be ten liters.

The wet test for fog is made by coating about twelve cubiccentimeters of the emulsion on a 5 by 7 inch plate, setting iton the cold slab, and placing it in a plateholder with the slidehalf drawn. It is exposed about eight feet from a z g-watt in-

SLOW EMULSIONS 91

candescent lamp for a few seconds, and then washed under theIap and developed in total darkness for about three minutes inany ordinary M.Q. developer. The exposed portion will de-velop up jet black, and the unexposed part should appear bycomparison dead white in the Mazda light.

E. J. WaIl gives the following formula for a process emulsion:

A. Potassium bromide. . . . . . . . . . . . . . . . . . . . 450 gPotassium iodide 5 gGelatin 625 g

Water 4800 cc

B. Silver nitrate 500 gWater 12 So ccAmmonia to redissolve

Solution B is run into the salts solution through a very fine jet,laking fifteen minutes. A is at 680 F. (20

0 C.), and B at1130 F. (450 C.). The mixed emulsion is then digested forliIteen to thirty minutes, cooled, set and washed, and finishedill the usual way. Note that the viscosity of the salts solutionis quite high. Jahr suggests at least doubling the gelatin used inemulsifying an ordinary type emulsion and increasing the iodideby fifty per cent.' It is the author's experience that an iodiderontent of not more than 1.5 per cent of the silver nitrate equiva-lent, a considerably lower concentration of the gelatin in the jar(about four per cent), and a very concentrated silver solution,give the best grain and contrast for photo-mechanical ernul-sions, provided always that the addition of the bulk gelatin andthe setting be very quick.

When we come to plates or films suitable for photogravure,where a density range of not more than 1.5 is required, and amnximum density of say 1.8 (the lowest tones having a densityof 0.3), a somewhat modified type of emulsion is needed, in-volving a longer scale of gradation and lower gamma infinity.This may be obtained in two ways. Either fifty per cent only


of the silver may be redissolved and the remainder added plainto the salts five to ten minutes after the first precipitation, andten to twenty minutes allowed for ripening, or a portion of thesilver nitrate solution, say one third, may be first added diluteto the salts, with the iodide omitted. Instead of redissolvingthis first silver nitrate, about one third only of the amount ofammonia that would be required to redissolve it, should beadded. This will, of course, give a dirty brownish black solu-tion containing un dissolved silver hydroxide, but this will notdisturb the result. On emulsification it will yield a perfect sil-ver bromide precipitate. After an interval of, say, three min-utes, the iodide is added in about ten times its weight of water,rendered alkaline by ammonia to about five per cent of the totalweight of silver; immediately afterwards the balance of silveris added plain, at a temperature of IISo to 1200 F. Ten tofifteen minutes further ripening may then be given, and thebulk gelatin added. The formula would thus appear as follows:

A. Ammonium bromide .Gelatin .Water .

Temperature r ro" F.

B. Silver nitrate .Water to .Stronger ammonia water .

Temperature 70° F.

C. Ammonium iodide .Water .Ammonia .

Temperature 70° F.

D. Silver nitrate .Water to .

Temperature 120° F.

E. Dry gelatin .

230 g160 g

2600 cc

120 g440 cc40 cc

6 g60 cc18 cc

240 g960 cc

SLO\i\l EMULSro S 93

In coating emulsions for photo-mechanical work, it must beborne in mind that the thinnest possible film should be madewhich is sufficiently robust. A considerable amount of han-dling takes place in processing, and while the film must exhibitIlO tendency to frilling and must be abrasion-proof, it must beIhin enough to make the various operations such as cutting withIvrricyanide, intensifying, washing, etc., as quick as possible.The addition of a yellow dye such as Brilliant Yellow, Tartrazin,or aphthol Orange, will assist in preventing spreading of theimage by irradiation. Some form of backing is, however, themost satisfactory means of preventing halation.

The slow photo-mechanical emulsions above described rangeill speed from 25 to ISO Hand D. A still slower type is usedfor black tone diapositive and lantern plates. A speed of 5 Hnurl D is very suitable for lantern emulsions, and here again:11 isolute freedom from fog is most important, as it greatly en-hances brilliance on projection on the lantern creen.

The formula below is stated to yield excellent slow iodo-hromide emulsions for transparencies:

A. Potassium bromide .Potassium' iodide .Gelatin .Water .

B. Silver nitrate .Water to .

ISO g

4 "250 g

2500 cc

375 g2500 cc

()IIl' cubic centimeter of five per cent pure hydrochloric acid is.uldcd to A. Both silver and crock are heated to 1300 F.( So.) and emulsified. The crock is stirred and maintained atI 10° F. (600 C.) for thirty minutes. Then add 300 g of dryVC'la I in and dissolve with stirring. The emulsion is then setIIIf in ice water. While the addition of a little ammonia to therruulsion is suggested just before setting, such procedure is notI('('ol1lmended by the author. The washed emulsion, after the


addition and solution of another 250 g of dry gelatin, must bedigested at I200 F. (490 C.) for a time varying from an hourand one-half to three hours. This time depends largely on thebrand of gelatin used, and can be decided only W actual test.The usual procedure is to take samples periodically from thecrock as it stands in the digesting pan, and coat sample plates,making sensitometric tests of the series of plates when dry underidentical conditions of exposure and development. Finals are:

Spirit $00 cc$ per cent chrome alum solution . . . . . . . . . . . . $0 cc

10 per cent potassium bromide solution .... . . $0 cc

As a preservative in this or any slow emulsion, there is little·to beat carbolic acid (phenol), of which about three grams tothe gallon may safely be used. This may be dissolved in thespirit, except where the spirit is poured on the shreds in re-melting them after washing. The final bulk of the above emul-sion prior to filtering should be I:i ,500 cc.

Reference to the literature will disclose many formulas inwhich the gelatin is divided equally between the salts and thesilver, thus mixing two viscous solutions in the precipitation.While an extremely fine grain can be obtained in this way, itis not easy to avoid grain aggregates, which give black" pep-per JJ spots in the developed image. Preferable to the divided-gelatin method is the double-jet method of precipitation de-scribed later.

A slow emulsion made with a mixture of the chloride andbromide of silver has the property of yielding warm black, sepia,or reddish-brown tones, on straight development, and has there-fore been much used for transparency and lantern slide work.Similar emulsions applied to paper give warm black tones whichhave been much in vogue. These tones are to be distinguishedfrom the tinted images obtained by the use of motion picturepositive stock coated on slightly tinted nitrate base.

SLOW EMULSION S 95

The ratio of bromide to chloride is capable of infinite varia-Iion, affecting both contrast and calor, although the physicalformation of the halide grains can equally influence both theIon formed on straight development and the effect on tone ofn-strained or diluted developers. Just as the higher affinity ofiodine for silver over that of bromine will cause an iodide toturn out some of the bromine of silver bromide, so a bromidewill convert some of the chloride of silver into bromide or bromo-r hloride. If the bromide be much in excess of the chloride in,In emulsion in which both halides are present, the tone willhI' 'older but the gradation will usually have a longer scale; an(' 'Cc s of the chloride will tend towards higher contrast and a, horter scale, but will more readily yield color on suitable de-velopment,

A point noticeable in the study of published formulas is thatill these emulsions a very small excess only of soluble halideuvcr the combining weight is employed, and that free acid, oftenIII considerable quantities, is used. This will be seen from theIwo following formulas, due respectively to J. B. B. Wellington,1IIc! E. Valenta:

A. Ammonium' bromideAmmonium chlorideSodium chloride .Citric acid .10% nitric acid .Gelatin , .Water · ..

B. Silver nitrate , ..Citric acid .Water .

Wellington20 g

Valenta2.0 g

lI.6 g

4 cc65 g

$00 cc

$0 g

$00 cc

10 g$0 g

70 g$00 cc

$0 g50 g

$00 cc

'I'lu- method of mixing varies. In Wellington's formula, thet1WI" olution is introduced in a fine stream at room tempera-

1111(' to the salts solution, which has been heated to I600 F.

PHOTOGRAPHIC EMULSION TECHN IQUE

(700 C.). The emulsion is allowed to stand for ten minutesand is then set and washed. Valenta on the other hand rec-ommends heating both salts and silver solutions to 1400 F.(600 C.) and adding silver to salts in the usual waY.

Another formula, due to Valenta, and suggested for trans-parency work, is quoted by ] ahr: 2

A. Ammonium bromide .Sodium chloride .Gelatin .Water .

B. Silver nitrate .Water

r5·2 gI.5 g

50 g400 cc

30 g........... '" . ' " 400 cc

Both solutions are heated to 1400 F. for emulsification, andafter mixing, the jar is maintained for an hour at 1400 F., beingmechanically stirred during this time. It is then set and washed.

It will be noted that after washing, these emulsions have arather low gelatin concentration. Chloro-bromide and chlorideplates should have a very thin film of emulsion and a coatingweight of not more than twenty-five to forty milligrams of silverhalide per square decimeter. If difficulty is experienced in coat-ing in the laboratory, a little fresh gelatin may be added to thewashed emulsion on remelting, bringing the gelatin up to aboutsix per cent of the total weight of liquid. It must be verythoroughly dissolved by.stirring, solution being complete be-fore adding the alcohol and chrome alum. It is usual to resetthe emulsion after making up and to remelt just before filtering,prior to coating. Little if any free bromide should be added'a small excess of chloride may be beneficial, although it may im-part a greenish tint to the blacks, or be prejudicial to warmbrown tones by imparting to them a greenish tinge. This greentinge is particularly difficult to eliminate in chloro-brornide pa-pers. If there is a tendency to fog, or if the image is too fiat,a little potassium chloride solution may be used, together with

SLOvV EMULSIONS 97

,1 little citric acid, not potassium citrate. Better still is a changeof gelatin. Most manufacturers are aware of the particulartype of gelatin which suit these slow emulsions, and will pro-vide likely samples if asked.

Potassium chloride is suggested rather than sodium or am-monium chloride, as it is not hygroscopic, and hence can beused even with an unwashed emulsion. It will be noted thatthe chlorides and bromides of various alkali metals are used indifferent formulas, but experience has proven that provided therespective combining weights are employed they all work muchalike. This does not apply to metallic salts such as cupric chlo-ride and thallium chloride. The use of one to three per centof cupric chloride (compared with the silver) can perceptiblyincrease the brilliance, tending to give improved whites andbetter contrast in projected screen images. Cupric chloridesolution is added to the finished emulsion when making up.

The gelatin being at high concentration in emulsifying andthere being practically no excess of soluble halides and a lowpl I, little ripening takes place, hence the grains remain smalland any tendency toward speed is inhibited. According to[ones," redness of the image, i.e., its departure from black, isdependent on the size of the developed grains. This was wellillustrated by the work of L. F. Davidson and T. T. BakerOil the photomicrography of developed chloro-bromide imagesusing a :fo-inch oil immersion lens (Beck) and dark groundillumination, with Agfa.color plates. The color of the grains byIransmitted light was excellently recorded. Warmth of toneis best regulated by length of time of reduction coupled withIh presence in the developer of a small trace of silver halidesolvent. The particular type of fine-grained emulsion contain-ing chloride in conjunction with bromide of silver seems to lenditself to the production by direct development of colloidal silverof ultra-microscopic size, yellow, red, or purple grains being re-

PHOTOGRAPHIC EMULSION TECH JIQUE

duced with a proportion of black grains of microscopic size..The presence in the developer of ammonium chloride or am-monium carbonate definitely helps the formation of color, butthese compounds can only help when the grain size is of a suit-able fineness. Apart from the actual size of the silver halideprecipitate, the use of more concentrated gelatin in emulsifica-tion gives a more solid clothing of the particles, which againappears to assist in the production of color. In the case ofpure chloride emulsions (q.v.) it is noteworthy that a plainhydroquinone-caustic soda solution, or a developer such as theGevaert G-25I, will yield most ea ily a series of tones fromwarm black to red merely by dilution, giving longer develop-ment time with exposure remaining more or less constant. Sev-eral hours' development with extremely dilute hydroquinone-soda can be made to yield purple and bright magenta tones.

Silver chloride darkens in the light, but will yield only a flat,grayish image on prolonged exposure." Given an exposure un-der a negative of the order of 25,000 meter-can dle- econds,which mayor may not give rise to a faint visible image, theemulsion can be developed to an intense blue-black in any suit-able solution such as amidol or metol-hydroquinone. Silverchloride is, in general, emulsified with gelatin by adding silvernitrate to the solution of chloride, though the pouring of thegelatin-salts solution on to the silver solution has been advo-cated. The gelatin is sometimes dissolved in the salts solution ,sometimes divided between the silver solution and the salts solu-tion. Another favorite method is to run solutions of silvernitrate and sodium, potassium, or ammonium chloride throuzh

b

* If a small quantity of potassium or sodium nitrite be added to asilver chloride emulsion, it will " print out" in sunlight and give an imageof sufficient depth to be toned with gold and fixed. Exposure-meterpaper is usually prepared with a nitrite to induce visible darkening of thesilver halide.

SLOW EMULSIO S 99two matched jets into a plain solution of gelatin, the salts beingalways very slightly in excess. This method has the advantagethat the silver chloride is not formed initially in a large excessor soluble chloride, which excess diminishes progressively as theprecipitation proceeds, but the conditions of colloidal precipita-tion are maintained constant through the entire emulsification.

One essential of a chloride transparency or development pa-per is that the whites must remain absolutely clean, that is, thatt he fog density on normal development does not exceed abou t0.02. Another is that the blacks are bluish black or warm black,but are not greenish in tone. These features are mentioned be-cause it is by no means easy to insure them. Also, as with allslow emulsions, it is not always easy to obtain sufficient con-t rast. A gamma of 4.00 or thereabouts should be one of thefeatures of a chloride emulsion. One difficulty in the makingor the emulsion is the avoidance of coarse precipitates, whichdevelop spontaneously owing to insufficient colloidal protection;t h se cause the black spots or flocks previously referred to aspepper, and may easily spoil an otherwise good emuI ion.*Their formation appears to be quite independent of the con-centration of gelatin or rate of stirring, but it is always a liabil-ity where the chloride of silver is concerned. A suitable coatingweight is twenty to twenty-five grams of silver chloride perliter, one liter being allotted to a twenty-foot run of paperIorty-two inches wide or the equivalent of four hundred lanternplates 3 by d· inches.

Many formuias have been published for fine-grain, slow emul-sions, particularly chloride and chloro-bromide, in which the

* A certain amount of small flocks has been found an actual advan-tll~C in emulsions for photogravure. Their formation appears to robIhe blacks of some of their density, thereby making them more trans-par nt and assisting shadow detail, while not interfering perceptibly withhighlight detail.


salts and silver solutions are poured simultaneously into thecrock containing plain gelatin solution. The silver halide isthen formed throughout the precipitation with a constant ex-cess of salts, which can be adjusted to be extren ely small, andcan be controlled by the time of lead given. Thus in Fig. 25

FIG. 25

the two solutions are poured into two cylindrical funnels theends of which have been drawn out in the flame into fine jets.By making the drawn-out part long enough.(Fig. 26) mall bitscan be filed and broken off in different positions as indicated bythe arrows until the cylinders empty at the same rate. Anothermethod is to connect a short length of barometer tubing, sayone inch long, to the outlet of the funnel by a piece of rubbertubing; in this way tubes of the same bore can be assured, andthe salts merely started, with a pinchcock, so many seconds be-

SLOW EMULSION S 101

fore the silver. Any timing must of course be made with sam-ples of the actual solutions to be used, each at its respectivetemperature. It is not advisable to use funnels with glass taps.If say 2000 cc each of silver and salts are to be poured into, ,

FIG. 26

5000 cc of ten per cent gelatin solution, 25 to SO cc of the salts~nll1tion would be run in before the silver was started. Thiswould insure a lead of halide and prevent free silver nitrate fromIoming into contact with the gelatin, which might give rise todichroic fog or veiling.

'fixing of this type, as a rule requires vigorous stirring. Thismay be done by means of a glass, wooden, nickel, or chromium-pial cl propeller, or a pair of propellers running in opposite di-11'(Iions. Small variable laboratory stirrers are supplied byl<:ill1r and Amend and Central Scientific Company; these are1111('(1with induction motors so that no sparking takes place.11 will be found, however, that for quantities of two liters up-w.irds, with viscous solutions of gelatin, considerable power is11'<]11ired to insure efficient continuous stirring. With sufficient111;11Iice the hand and wrist and a one-half or three-quarter inch1IIIing rod are difficult to beat for experimental lots of aliter

III Iwo. But where a series of strictly comparative tests is being111,1111',Iroper control and uniformity of mixing is naturally es-.1IIIial. In works practice a one-ha1f-h.p. motor would havellill('i nt power for stirring a crock of three gallons capacity

III III()re.


It must be borne in mind that starting with 5000 cc of tenper cent gelatin solution, the gelatin concentration in thecrock at the end of mixing will have dropped to 5.5 per cent.If uniform concentration is to be maintained, 1Jten if x percent of gelatin be used in the water in the crock, the silverand salts solutions must each contain x per cent of gelatinalso; in this case the gelatin concentration of each of the threesolutions would be reduced to 5.5 per cent in the above example.Further reference to this type of emulsification will be foundin Chapter VIII.

Care must be taken in the drying of transparency plates, asowing to the thinness of coating, drying marks at the edges areapt to appear. The rate of drying is limited by the rate ofdiffusion of water in the gelatin layer to the evaporating sur- ~face; 4 it thus progresses more rapidly at corners and edges andproceeds by a gradual regression of the moist area to the centerof the plate or film. It would appear that the extra tension dueto the more rapid drying renders the sensitive salts liable todarken on development without exposure (cl. pressure marks),and to prevent the trouble with thinly coated slow plates, it willbe found advisable not to turn the heat on in the drying cup-boards as soon as in the case of thicker-coated bromide plates.An addition of one-quarter per cent of glycerin to the emulsionmay be found helpful.

Chapter References

1. Twentieth Century Fox Film Report, 1933.2. R. j ahr , Handbuch del' Wiss. und Angew. Phot., p. 240.

3· Chapman Jones, Phot. J. SI, pp. 159-174.4. Photography as a Scientific Implement, p. 139.

CHAPTER VICOLOR-SENSITIVE EMULSIONS

Action of Color-Sensitizers - Orthochromatic and Panchromatic Plates.md Films - Self-filtering Emulsions - Hyper-sensitizers - Bipacks andTripacks - Three-layer Col or Films - Dye-couplers and Color Formers

A PLAIN silver iodo-bromide emulsion is not sensitive tocolor as is the human eye. The maximum response is

I() light of wavelengths ranging from 430 to 470 mu in the blue-violet, the sensitivity ending at about 520 mu in the green region(If the spectrum. Ordinary emulsions are sensitive to the ex-l rcme violet and ultra-violet, response to the latter being ter-urinated by the ultra-violet absorption of gelatin, By reducingIh(' gelatin to a minimum, the response can be lengthened to.rhout I80 mu. Silver bromide emulsions are also sensitiveIII the shorter radiations having wavelengths varying from10 " to IQ 7 cm, and even to the gamma rays, which are of theurder of 10-8 mm,

The response of a .gelatino-bromide emulsion to the spectrumIIf white (incandescent) light is seen in E in Fig. 27; the visual,11 Iivity V is shown in comparison with it. It is seen that while1111' most active color visually is situated at about SS 5 mu, theIlI'CIIl1ideemulsion has a maximum response at a much shorterw.rvclength. The relative sensitivity of average" ordinary"III "('olor-blind" emulsions, orthochrornatic (green-sensitive),.uul panchromatic (fully color-sensitized ) types is seen corn-".III-d in Fig. 28, the green-sensitive type (B) having a charac-11'11 Ii ' hump at 540 to 550 mu, and the panchromatic (C) hav-11 • u ually - two less clearly defined maxima at 540 and

" ID 1111t, indicating the use of two color-sensitizers, Cornbina-


r:""

/' r"\If \ / \

\ / \ -I £ IV \I \

1/1 <::1 '.<\ \-I \ I ~1 V '\1 /\ \

v ' '\./ \. <;

300 600500 700 mfL

FIG. 27

tions of color-sensitizers have been found which together pro-duce an almost continuous response without appreciable max-ima from the violet to the middle red of the spectrum.

Color-sensitizers are dyes which when added to the emulsionactually color the silver halide grains or their sensitive nucleior spots so that they absorb additional radiations, the energyof which can be converted into chemical energy and thus renderthe grains reducible in the developer after exposure. As thephoto-chemical reaction when light impresses the film can onlytake place if the radiations are absorbed by the system, it canbe understood that the dye used must be complementary incolor to the rays for which it "sensitizes." The maximum ofsensitivity conferred does not, however, coincide exactly withthe absorption maximum of the dye, but in the case of iodo-bromide emulsions is generally shifted IS to 20 mu towards thered end of the spectrum. This shift is complicated by the factthat the dye may impart to the grains a different color from itsown in aqueous solution, and that it may color silver bromide,

COLOR-SENSITIVE EMULSIONS ros

silver chloride, and silver iodide, differently among themselves.An important desideratum in color-sensitizing is to provide asfar as possible that no excess of dye is used over and above thatactually needed to give the optimum result in any particularemulsion.

For many years after the possibility of color-sensitizing hadbeen discovered, only greenish-yellow sensitivity could be addedlo the normal blue-violet response. This was done by addingthe dye eosin to the emulsion, first suggested by Waterhouseand afterwards used successfully with collodion by Vogel.Tailfer later used eosin and erythrosin in conjunction with am-monia in making gelatino-bromide plates. Erythrosin is one ofthe finest green sensitizers available even today, and is an alkalisalt of fluorescein (potassium or sodium tetra-iododofluorescein-at ). It stains the grains pink or magenta, so that they absorbIight of the complementary color green; the dyed emulsion isthus sensitive to violet, blue and green. The increased truthful-Il(,SS of the monochrome rendering of colored objects with theseplates caused them to be named orthochromatic or isochromatic.Wall states 1 that from 0.07 to O.IO gram of the potassiumsalt is generally used to one hundred grams of silver nitrate.

In the first years of the present century, fairly powerful yel-low and red sensitizers were discovered, anel thus it became pos-sible to render an emulsion sensitive to the entire range of the

~, .,,:,-, ,. <,-i-oI:::'./ , --/ \ '. - ,

Y·. . '. J.l, \i.1 \ '.' • \

\ '. ,

2

J

~IOO 400 600.soo

FIG. 28

106 PHOTOGRAPHIC EMULSIO TECHN IQUE

visible spectrum. Such emulsions were termed panchromatic.It must be noted, however, that comparatively few dyes willact as color-sensitizers, and that these require to be used in thehighest possible state of purity if consistent resul are to beobtained.

Two things will be noticed from a consideration of Figs. 27and 28. The visual and photographic curves of Fig. 27 aresimilar in shape but considerably displaced as to wavelength,Fig. 28 shows that while the blue-violet sensitivi ty A remainsin each type of emulsion, neither the ortho B nor the panchro-matic variety C has anything like the respon e of the humaneye to the other colors. Color-sensitive plates, from the emul-sion maker's standpoint, must therefore be divided into threedistinct classes:

(c) The orthochromatic type, suitable for zeneral landscape and flowerphotography, document work, etc., when used with a yellow filteron the lens to depress the predominant blue-violet sen itiveness.

Cb) The panchromatic type, more or less evenly sensitized to all vis-ible colors from 400 to about 650 m!" required for black-and-white reproduction of colored objects fully corrected, or for colorphotography where the spectrum is divided into three more or lessequal parts, blue-violet, "Teen and orange-red.

Cc) Locally sensitized types, with sharp peaks 01 color-sensitiveness, ofspecial value in conjunction with contrast filter Ior photornicrog-raphy or for regional spectrography and other scientific purposes;in this category infrared plates would be included.

In class (a) the predominant sensitivity to blue-violet may bereduced by the inclusion in the emulsion of a yellow" filtering"dye. Such a dye should not have any desensitizing effect onthe intrinsic speed of the silver bromide grains. Naphthol yel-low, tartrazin and thiazol yellow have been recommended; 2

brilliant yellow is another suitable dye. The filter-dye methodcan also be made to depress the blue-violet in a panchromaticemulsion to make it have a color response similar to curve V

COLOR-SENSITIVE EMULSIO S • 107

in Fig. 27, but by some curious mischance this has never beenoffered to the public by manufacturers, who appear to find thepanchromatic film plus compensating filter more practical. Anemulsion which gave remarkably good monochrome renderingof colored objects without a filter was that in which F. F. Ren-wick used auramin," which he claimed not only acted as a bluesensitizer but increased the range of sensitivity in the red ob-tained with pinacyanol; the auramin actually depressed violetand made a partial self-screening, though this is not claimed.

The green sensitizer used in the most popular orthochromaticemulsions is undoubtedly erythrosin (Fig. 29). It is largely

'Ic)O 520

FIG. 29

600560

t-mployed in the industry for plates, films, and document papers,and is used in many rollfilm emulsions where it confers mildgreen sensitivity and increases brilliance. It gives little colorcorrection in daylight unless used with a yellow filter. Eryth-rosin can be added to a finished emulsion or it may be mixedwith the silver solution just before emulsification. Eder's earlyformula, which has never been greatly improved, involved theaddition of a solution of erythrosin to the ammonia-redissolvedsilver; this procedure probably gives the highest peak of greensensitivity that can .be obtained. The addition to a finishedr-mul ion of a solution of erythrosin containing a small quantityof ammoniacal silver hydroxide or a weak solution of silverchloride in ammonia, may give extremely pronounced green


sensitivity, but is likely to cause fog, poor keeping, and blackspots, and is only advised where a very pronounced peak isdesired for special work.

The following emulsion, based on Eder's lines, will be foundvery satisfactory:

A. Water .Ammonium bromide .Ammonium iodide .Soft gelatin .

3500 cc350 g

8 g15o g

B. Silver nitrate 500 gWater 250 ccAmmonia, concentrated to re-dissolveJust before mixing, add two per cent crythrosin,dissolved in equal parts of alcohol and water 25 cc.

Using the salts solution at 90° F. and the silver at 70° F., thesilver is "flopped" into the crock without stirring. Whenadded, it is stirred gently for one minute. Six hundred andfifty grams of dry gelatin is then added and stirred in for fifteenminutes, bringing the temperature of the crock up to 105° F.It is then set quickly in ice water, very thoroughly washed afterabout five hours, and digested as usual. Final volume to beten liters, inclusive of:

5 per cent chrome alum solution lOO cc5 per cent phenol in alcohol 500 ccI per cent ammonium bromide solution. . . . . . 50 cc

If the emulsion is to be made self-screening, there should beintroduced before the finals sufficient of the yellow dye in awatery solution of about two per cent strength to impart a paleyellow calor to the emulsion when tried out on glass coated withabout the normal thickness, la or 12 cc to a 5 by 7 plate. Anumber of trials must of course be made, and tried out whendry by photographing a calor chart in daylight or daylight-filtered Mazda without a compensating filter. Loss of peed

COLOR-SE- SITIVE ElVIULSIO S 109may be due to the presence of a " filler" in the dye, or to theuse of too large a quantity of dye, or to insufficient greensensitivity to admit of the use of the filtering dye. Pure dyesmust be used for this purpose.

Erythrosin for emulsion-making purposes is made of excep-t ional purity by Meister, Lucius and Bruning of Hoechest am

fain, Germany. A pure grade is also supplied by EastmanKodak Company (product of National Anilin Company).Commercial erythrosin may be purified, according to Wall," bydi solving three parts of the dye in one hundred parts of water,and adding dilute sulphuric acid gradually until the whole ofthe tetra-iodo compound is precipitated, the solution being thencolorless, The precipitated dye is filtered off and washed re-peatedly with distilled water. It may then be convenientlymade up as a two per cent solution, dissolving it in a mixtureof equal parts of distilled water and alcohol, to which a fewdrops of ammonia have been added. Instead of adding theerythrosin to the ammoniacal silver solution, this dye solutionmay be added, as stated, to the finished emulsion, in which casethe quantity employed would be the equivalent of about onepart of solid dye to one thousand parts of silver halide. It isbest added five or ten minutes before the end of the final di-gestion. It must be remembered that any emulsion sensitizedfor greenish-yellow must be handled in deep red light. Bothemulsion and coated plates should be shielded from direct rays,and the safelights should be re-tested from time to time.

While there may be nothing to replace the old erythrosin asan all-round green sensitizer, Meister, Lucius and Briining haveproduced pinafiavol, which confers extraordinary sensitivity inthe blue-green, continuing through to the beginning of the yel-low. It appears to show a tendency with some emulsions togive fog, and a somewhat low gamma. It is a basic dye, andthe fact that it produces a response to the most obstinate part


of the spectrum, the blue-green region around 520 mfA.,makesit of particular interest. The piriaflavol maximum is, in fact,at about 527 to 530 mu. For adding to a finished emulsion,the makers recommend the use of about 20 cc of a : IOOO solu-tion to the liter. Both Orthochrom T and pinaverdol are rec-ommended, but while excellent green sensitizers, they confersuch high orange and red sensitiveness that they should be con-sidered along with the panchromatic dyes.

The use of too much dye is liable to cause fog, does not in-crease color-sensitivity, and may slow the emulsion by deeplycoloring the grains and the surrounding gelatin, thereby actingas a light-filter rather than as a sensitizer. This point is wellillustrated by the increase in sensitivity which can be producedby merely bathing commercial panchromatic plates in plainwater.

The next type of dyes to be considered comprises the deriva-tives of quinoline, such as cyanin it elf, the iso-cyanins intro-duced by Miethe, and the carbo-cyanins more recently appliedby Konig. An immense number of these sensitizers has beenproduced and investigated in recent years. It is beyond thescope of this book to enter into the chemistry of these com-pounds, and for their study the reader is referred to the lastfew years' issues of (( Photographic Abstracts," published quar-terly by the Royal Photographic Society of Great Britain, andthe library of the U. S. Patent Office, where a vast and con-stantly growing number of patents are available. Practicallyevery important manufacturer of films and plates today pos-sesses his own organic laboratory for the manufacture of hissensitizers. A few of these have been placed on the market, butthe firms concerned have not been over generous, and the creamof such products are not only reserved for factory production,but their use is hedged around by the patent position.

The great increase in speed of panchromatic emulsions wit-

COLOR-SENSITIVE EMULSIO S III

ncssed in recent years has been largely due to the new techniqueof (( uper-sensitization." That it is possible to attain sensitiza-(ion by the addition of such super-sensitizers higher than the sumtotal of the individual sensitizers, is claimed by Dr. C. E. K.i\1 es." An alcoholic solution of the sensitizer and super-scn itizer is diluted with water and added to the emulsion. As:t given instance, ten parts of pinacyanol may be super-sensitizedwith one part of pinafiavol, the region of response induced byeach overlapping. The net effect is to obtain greater speed in, ....the ensitized region than the sum of the two sensitivities con-ferred by each separately. Another example is that of a(hiacarbocyanin, super-sensitized by the addition of 8-alkyl-(1 iabenz th iacar bocyanin.

The amount of the e color-sensitizers added to an emulsionis small as compared with fiuorescein salts. About 0.05 per centIlf the weight of silver bromide would be used in the case ofpinacyanol; 0.07 per cent in the case of pinachrome, OrthochromT or pinaverdol. These dyes are all rapidly decolorized on ex-posure to light in the form of aqueous solutions or thin films.The color-sensitiveness they impart is retarded by the presenceof free bromide in the emulsion, which appears to oppose thedyeing of the halide grains. This is probably the ~~~on whythe addition of a trace of silver nitrate to the sensitizing bathoften increases the effect, but such addition is dangerous practiceunless the washing of the emulsion is made under the strictestpossible control. , .

orne of the modern red-sensitizer give the best results Ifadded to the emulsion before it is completely digested, this seem-ing to a sist in the adsorption of the dye on the halide grains or(heir nuclei. Whether the emulsion is digested with the dye in,or the dye is added at the final stage, it is important that itshould be ab olutely free from fog - one for example whichwill give on normal development without exposure, a fog read-

II2 PHOTOGRAPHIC EMULSIO TECHNIQUE

ing of not more than 0.02. For the reason already noted, thesmallest possible quantity of free bromide should be added asa final, and extra careful washing should be given to insure thatthe excess bromide used in emulsification is thoroughly removed.Lithium bromide is sometimes recommended as giving cleanerresults than the potassium salt. An emulsion having a lowiodide content, say two per cent only of the silver bromide, alsoreacts in general more favorably to the sensitizers. Owing .totheir fugitive character the isocyanins and analogous dyes shouldbe made up in artificial light and stored in dark bottles. A warmmixture of ethyl and methyl alcohol is generally used to dis-solve the dye, which is as a rule soluble only about one part in .a thousand (I: 5000 in the case of Sensitol Violet), and whendissolved the solution is broken down to strength by furtheraddition of alcohol. Where two or more sensitizers are used, itis advisable to add each one separately, with a short interval be-tween additions.

Different sensitizers must be used in different quantities toproduce the best results. In the majority of cases the spectralband or bands for which the dye sensitizes is not interfered withby the next dye added. Thus it is claimed that pinafiavol, usedto sensitize up to 590 mu with a maximum at 527 mu, can beused with pinacyanol, which will continue the sensitivity alongthe spectrum up to about 640 mu, By a combination of dyesthe peaks of which, as measured on a wedge spectrograph (p.2I0), add up to a more or less uniform level, it is possible toproduce a panchromatic emulsion having almost equal sensitiv-ity throughout the visible spectrum.

For many cases of scientific work, it is of advantage to useplates or films locally sensitized; thus eosin, erythrosin, rho-damin, etc., give a pronounced green maximum and can be usedwith good results in conjunction with a green contrast filter inthe photomicrography of specimens stained with carbol fuch-

Violet Blue Green Oronge Red

PANCHROMATICTYPE A (Tungsten)

/

.4••••••••••. "r.;t~:lnll'llIIIl.~,.,.""\.,1l"I'I'..,~PANCHROMATICTYPE A (Daylight)

PANCHROMATICTYPE B (Tungsten)

PANCHROMATICTYPE B (Daylight)

PANCHROMATICTYPE C (Tungsten)

PANCHROMATICTYPE C (Daylight)

FIG. 30. TYPES OF PANCHROMATICEMULSION, A, B, ANDC. (Courte:;;/1 t Kodak Co.) Visible violet begins just before 400 mll (40 on t.as man . ivit d t aboutscale) , highest visible green is at 550 mll, and red sensitivi y en s a

Ilso mll, although the eye responds up to 700 or beyond.


sin, etc. On the other .hand, dicyanin is a good sensitizer fordeep red, leaving an in ensitive gap in the green; such an emul-sion is useful for work with a red or orange contrast filter forphotography of designs, wood grain, photography Jhrough mist,and so on. Where work is required mostly in the oranze region,and deep red sensitivity is not required, Orthochrom T, pina-chrome and pinaverdol (Ilford Sensitol Green), can be recom-mended. Tewer dyes, especially for the infrared region, areto be found in dicyanin.: above mentioned, kryptocyanin andneocyanin.

The sensitivity of some panchromatic plates as manufacturedby the Eastman Kodak Company is seen in Fig. 30; these rep-resent the A, B, and C types of material. It will be seen thatall of them exhibit the inherent sensitivity from the ultra-violetup to 500 mu, but that the ratio this region bears to the regionof longer wavelength varies. The upper spectrogram in eachcase represents the response to incandescent electric light, thelower one to daylight. Incidentally, these pairs give an excel-lent idea of the difference between daylight and artificial lightas regards spectral distribution. It may be noted, too, thatthe red-sensitivity appears to extend further into the red in thecase of incandescent light, but this, of course, is due to thegreatly increased energy of the latter in the red region. As ex-plained in the chapter dealing with sensitometry, the spectralcharacter of the light must be taken into strict considerationin making any measurements of either the speed or the color-sensitivity of panchromatic emulsions. Many of the spectre-grams published in the early literature are entirely misleadingat first sight, because they were made with artificial light, oftenincandescent gas, and for the same reason any speed measure-ments of calor-sensitive materials can be accurate only if thelight sources by which they were obtained are specified. Theundyed emulsion is sensitive up to 500 mu, let us say. We then

COLOR-SE SITIVE ElVIULSIOl S II5

add a new band of 500 to 600 mu by means of a color-sensitizer10 the response. Assuming the useful region in each case tobegin in the near ultra-violet at 350 mu, we have a responsein the plain emulsion of 350 to 500 m[-t,and of 350 to 600 muin the other; hence the area of response in the spectrum is five-thirds as great in the case of the sensitized emulsion as in theplain one, and its speed will appear proportionately greater if ithe exposed in light which is rich in the added wavelengths.

The speed of a color-sensitive emulsion can thus only beestimated fairly by taking into account the spectral quality ofthe light. The speed numbers given by some manufacturershave in past decacles been very misleading, because if correctcalor rendition of the blue-violet in monochrome is required inIhe studio, and the necessary filter be used on the lens to obtainit, the effective speed will be reduced again to what it wouldhave been had the speed measurement been made by daylight orproperly filtered tungsten light.

We now come to the infrared region of the spectrum, invis-ible radiation of wavelength extending from about 760 mu on-wards to lOG mu. The early infrared region has recently be-ome of con iderable importance in aerial photography and in

much industrial work, both of which are making rapid advanceowing to the excellence of the infrared' material that has becomeavailable. A plain silver bromide emulsion of the boiled typewas prepared by Abney, and was claimed to be so red-sensitivethat a kettle of boiling water was photographed by its ownradiation. The author found that a collodio-bromide emulsioncould be sensitized for the early infrared with Benzo Green.Sensitivity up to 800 mu has also been obtained by bathingan ordinary plate for ten minutes in a five per cent solution ofsodium bisulphite, washing in water for a few minutes, andthen bathing again in a one and one-half per cent solution ofsodium carbonate. Infrared sensitizing is now obtained by the

II6 PHOTOGRAPHIC EMULSIO TECHNIQUE

ad~itiol~ of dyes to the emulsion. Dicyanin was used in I9I9,being discovered during the researches carried out during the19I4-I9I8 war to produce sensitizers that had previously beenmade only in Germany. Kryptocyanin was prodgced later, sen-sitizing for the region between 700 and 800 mu, with a max-

« «:YPTOC Y.A NtH

I

ENOCY.ANtN

fOO 600 700 too 'f00 7000 mf-l.

FIG. 31

imurn near 750 mu, It is a particularly interesting dye for ex-perim~ntal. work because it can be used with little fear of fog,and will. Yield plates of good keeping quality. Later, in I925,neocyanm was discovered, which extended the sensitive regionto 950 mu, One of the carbocyanins, xenocyanin, gives a max-imum at 960 mu and extends right up to about 1,200 mu, Cer-tain emulsions sensitized with it have a response up to 1,350 mu.Some of the responses of infrared sensitive emulsions are seenin Fig. 31. Tetracarbocyanins have been prepared by Dieterleand Reister c which are claimed to sensitize two or three timesas highly as the corresponding acetoxy-substituted dyes. A

COLOR-SENSITIVE EMULSIONS 117

method of preparing nonacarbo cyanins for infrared photog-raphy beyond 11,000 mu is described by these authors. Ac-cording to Clark," plates which are to be used for spectrographicwork beyond 900 mu must be hypersensitized in order to ob-tain practically useful speeds. These later infrared sensitizersare not at present commercially supplied, but are prepared bymanufacturers for their own use. Dicyanin, kryptocyanin andneocyanin, however, can be obtained.

In Fig. 32 is seen the most recent infrared introduction of theEastman Kodak Company, which is applied to plates and films

Courtesy Kodak Limited, Englal/4

FJG. 32. WEDGE SPECTROGRAM OF INFRARED PLATE AND FILM FOR AERIALPHOTOGRAPHY

ror aero work. It has a wide band of sensitivity, but when thisi restricted by the use of an infrared filter, the emulsion is statedto be many times faster than previously existing materials.

FILM PACK EMULSIONS.- Where separate emulsions areused in color-photography for the three primary images, platesmay be used in a split-beam or one-shot camera, or films maybe laid one over the other in the form of a sandwich and therays from the lens made to pass through the top film to the onebeneath it for the ~reen impression, and thence to a thirdfilm which receives the red impression. In such a sandwich,the top film is an un-color-sensitized emulsion, sensitive merelyto blue-violet.

For readier separation in a one-shot camera, three distinctemulsions are frequently used. One of these is panchromaticor red-sensitive, one orthochromatic or green-sensitive, and thethird normal or "color-blind." When three films are used in

lIS PHOTOGRAPHIC EMULSION TECHNIQUE

the form of a tripack, three similar emulsions are usually em-ployed. The most obvious order is an upper ordinary or blue-violet sensitive film facing the lens with a green-sensitive filmnext, screened with a yellow filter which may b incorporatedin the top emulsion or coated as a yellow filter layer on the baseof the upper film or used as a thin sheet of dyed gelatin betweenthe two films. This yellow filter also suffices to suppress the.

&00suo

FIG. 33

blue-violet in the back (red response) film if the latter is madered-sensitive only and not fully panchromatic, as, for example,if sensitized with dicyanin. If it be panchromatic, then a redfilter must be used. As there is already a yellow one screeningthe green-sensitive emulsion, the second filter may be magenta,since magenta + yellow = red.

Naphthol yellow or briIIiant yellow serves as a good firstfilter dye; azo rubine, crocein scarlet or rhodamin G will thenserve for the red screening component. The general scheme is

I

COLOR-SENSITIVE EMULSION S 119

seen in Fig. 33. The filtered responses of a commercial tripack(Defender) are shown in Fig. 34. In any attempt to experi-ment with tripack emulsions, it must be remembered that asthe three elements are exposed simultaneously, the relative fil-tered speeds of the three members must be equalized. The

j3I...U£. C'R.EE:N ~£D

600 700'100 s oo

FRONT FILM

FrG. 34

light is considerably diminished by its passage through the frontfilm, which must therefore have the highest possible transmis-ion, but the lowest sensitivity. The intermediate or green-ensitive film must also be sufficiently transparent to admit of

Ill(' necessary amount of light reaching the back (red) mern-bl'l'. The Defender tripack utilizes more heavily coated ernul-"ion for the blue- and green-sensitive components, so thathigher contrasts may be obtained; this is of course the equiva-lent of allowing a longer time of development for the yellow


printe~ in three-calor work. Alternatively, different types ofemulsion may be selected, the aim being that the three imazeso.btained .on development are of equal gamma and as far as p~s-sible of similar density range.

The following typical specification is made up from data ob-tained from a number of published patents dealing with tri-packs:

Front emulsion, Coating weight 40-$0 mg per dm".Average grain size 1.2-1.8 11-. Speed 80-120 Hand D.Percentage transmission to yellow light, 35-40%.

Middle emulsion. Coating weight 60-70 mg per dm>.Average grain size 2 11-' Speed 2$0-300 Hand D.Percentage transmission to red light, 30-35%.

Back fil'm. Coating weight IIO-130 mg per dm".Average grain size 2.$}J.' Speed 1000-1500 Hand D.

In assembling the Defender tripack, the two front films, blue-and green-sensitive, are placed face to face, with the base of theblue-sensitive film facing the lens. The third member of thecombination is faced lensward, shielded by a red coating onthe base of the green-sensitive middle film. Experimental packscan be most easily made up by using glass plates for the frontand back members and a film for the middle one. This fact ismentioned on account of the difficulty which may be experiencedin coating film base in the laboratory with sufficient accuracy.

The filtering of the blue-violet rays from the zreen recordingfilm, if not effected by the use of a self-screened emulsion can,be done with a very thin layer of gelatin stained with tartrazinor naphthol yellow between the two emulsions. As tartrazintransmits some ultra-violet, and aesculin or ultra-violet filtershould be used on the camera lens. If the red filter takes theform of an actual coating on the back of the green-sensitivemiddle member, a dye such as Congo red, which is easily de-colorized by an acid fixing bath, should be used. Carmine L,

!

COLOR- ENSITIVE EMULSIONS . 121

lanafuchsin SB, and tolan red are also mentioned by Wall forIhis purpose."

The use of a bipack is involved in two-color photography.Here the factor of the filter dyes and the speed of the emulsionsmust again be adjusted so that equal exposure is required ineach member for a white object. For two-color work in day-light the Eastman Kodak Company recommends Wratten fil-ler No. 28 and No. 4oA, or 0.28 and No. 40 for incandescentIungsten light. irnilar color analysis can be obtained by theuse of two films as follows:

Front. A fully green-sensitized emulsion, using a Wratten Aero No. I

"I' a K-2 filter on the lens.Hnck. A panchromatic emulsion.Interleaf. An orange filter between the two, or an orange coating on

IIH' base of the orthochrornatic film, both emulsion surfaces facing the

lvns.

The performance of experimental two-color packs can be prop-crly checked only by tests with the spectrograph (See Chap-1('1' XII).

The term monopack is used for a plate or film that comprises,1 series of non- eparable emulsion layers coated one on top ofIh(' other, the emulsions being self-screened to effect separation,'or dyed material being coated between the sensitive layers. Se-It'( live sensitization alone is notadequate. The idea dates back10 (891, when an English patent was granted to H. Kuhn. By, traigbtforward development, the appropriate colors are ob-t.riucd as silver deposits in each layer. These three black-and-while separation negatives are then converted into the respec-I ive color images either by the production of a dyestuff in theP:(l'ts occupied by the silver, or by destroying dyes already in-(orporated in the emulsion layers, or by dissolving out the nega-IIV(' images and color-developing the remaining (unexposed)ilver bromide.


In 1905, K. Schinzel, then a clerk but seventeen years old,coated a plate with three layers of emulsion, each layer beingsensitized for one of the three primary colors, but dyed withthe color complementary to it. Thus the red-sensitive layerwould be dyed cyan, the green-sensitive layer magenta, and theblue-violet-sensitive layer yellow. It was Schinzel's idea, afterdeveloping and fixing the exposed monopack, to treat it with.a weak solution of hydrogen peroxide. The dyes to be usedwere such that through the catalyzing action of the reducedsilver they would be decolorized to an extent proportional to thedifferent densities. Color formers instead of actual dyes wereshortly afterwards suggested by thi inventor."

The first Mannes and Godowsky patent 10 described an upperself-screened emulsion layer an top of a rapid red-sensitive emul-sion. This twO-COIOl·monopack was developed and fixed in theusual way, the reduced images being afterwards converted intosilver ferrocyanide, one for use as a mordant for a basic orangedye; this was done by means of a solution the diffusion of whichcould be controlled, The other (ferrocyanide) image was thentoned with a blue-green toning bath. From this brief descrip-tion, the beginning of the modern monopack can be appreciated.

A three-layer process was meantime worked out by Dr. BelaGaspar, whose prolific patents date from about 1930. Usingblack-and-white separation negatives obtained with a split-beamcamera, and making from them separation positives, he em-ployed for motion-picture printing in color, nitrate film stockwhich was coated on one side with a blue-dyed emulsion andon the other side with first a yellow-dyed, and then a magenta-dyed emulsion. The dyes were destroyed on development inproportion to the presence of developed silver; the image wasthen bleached and the residual silver bromide converted intochloride, on which the sound track was developed. The wholefilm was then fixed in hypo and all remaining traces of silver

COLOR-SENSITIVE EMULSIONS 123

removed, leaving a dye image of great brilliance. Such film wasof course applicable to printing only.

In a subsequent process," a monopack for exposure in thecamera is described. A blue-sensitive layer is coated on thesupport first, and this receives the lens image. On top of thisare coated the green and red layers. A distinguishing featureis that, to speed up the material, dye generators, rather thanactual dyestuffs, are used. The blue-violet emulsion, it is sug-gested, may contain about fourteen grams each of gelatin andsilver halide per square meter; the yellow (complementary) dyegenerator being 0.85 gram per square meter of r-phenyl-j-methyl-pyrazolone-g. The green emulsion layer is color-sensitized with 2-methyl-I-ethyl-pseudocyanin iodide, andcontains a dye developer of one gram per square meter of 1-amino-Senaphthol-j : 6-disulphonic acid, which is rendered in-oluble through precipitation within the emulsion by triphenyl-

guanidin acetate. This layer is made self-filtering by theaddition of 0.75 grain per square meter of tartrazin. The toplayer is color-sensitized with pinacyanol, using one gram persquare meter of diphenylguanidin acetate, and colored by a suit-able red dye. The film is developed, fixed, and washed, andthe dyes in the two lower layers generated by treatment witha nearly neutral solution of:

Water 100 ccSodium acetate 10 gDiazosulphanilic acid 0·5 g

This is used at a temperature of between 30 and 80 C.

The film, after being washed and dried, is treated with a bluedye solution, the dye being precipitated in the top layer. Thedyes are then destroyed in proportion to the silver deposits ofthe images by treatment with an acid solution of thiocarbamide.

In the Kodachrome process, after straight development of thethree primary images, the reduced silver is dissolved out and

I24 PHOTOGRAPHIC EMULSION TECH IQUE

the residual silver bromide is exposed and treated with a blue-green developer. An acid bleach is allowed to diffuse throughthe two upper layers, the lowest remaining blue-green. In thetwo upper layers the metallic silver is converted into silver chlo-ride. This is next exposed and developed with a magenta-forming developer. Then follows the next bleach which is con-fined by controlled diffusion to the top layer only, where onceagain the dye is destroyed (magenta) and the silver convertedinto silver chloride. This is exposed and color-developed yel-low, after which all that remains is to dissolve out the silverwith a solution such as Farmer's reducer (ferricyanide andhypo) .

It had been known at the time of the early experiments thatmany developers produced more or less in oluble dyes by inter-action with the silver deposit. Pyrogallol, the oldest known,gives orange, amidol yellowish-red, while blue dye images aregi;en by leuco-indophenols and leuco-indamines and indoxyl.Red images are given by the leuco-azomethines and thio-indoxyl. However, in alkaline (carbonate) solution they arereadily re-oxidized by atmospheric oxygen with the productionof colored fog. The chemical story of the investizations whichled to modern solutions of the problem is well told by Karl andLudwig Schinzel in their various articles in Das Lichtbild.

In any experimental work on monopacks, where dye genera-tors which are either insoluble or not diffusible, are mixed in theemulsion layers, the remaining unused generator must be easilywashed out; for example, with caustic alkali. The above au-thors 12 state that for color photography by simultaneous colordevelopment, coupling components which give the appropriatedye in each layer with the oxidation products of a suitable de-veloping agent or by simultaneous oxidation by the silver halide,must be mixed in the three layers.

The insoluble salts of the naphthols or their derivatives and

/

COLOR-SENSITIVE ElVIULSIONS

analogues, the arylsulphonarnines, pyrazalones and acidic meth-ylene compounds can be added to the layers. Calcium orbarium hydroxide, guanidin or sodium glycocollate may beadded to maintain stability. The carboxyl or sulphonic acidsof couplers may be added to the layers in the form of insolubleor barely diffusible salts with inorganic or preferably colloidalorganic bases. The coupling component is used in quantityequal to about one-half of that of the silver halide present. Adeveloper containing one per cent of p-amino-dimethylanilinhydrochloride and five per cent of sodium carbonate is rec-ommended.

One of the difficulties in tripack manufacture is the thinnessof the emulsion layers, which is indispensable in order to avoids atter. Present day precision coating machines are capable ofcoating layers which when dry are as low in thickness as 3 to4 fl. Optical contact, i.e., the actual coating of each layer ontop of the other, is necessary and the three coatings of a mono-pack must provide definition which it is impossible to obtainwith any form of tripack in which three separate films, how-ever thin, are employed.

The chemistry of direct and indirect dye couplers and of colorformers has already given rise to a literature of its own, andit is impossible to do more than touch on some of the aspectsof the monopack and its problems, in these pages. Recourseto the patent literature is necessary to any form of study, thoughan immense amount of information will be found in the articlesof the Schinzel brothers already mentioned.

For the purposes of experiment, when two (or three) layersof emulsion have to be processed, an addition of alcohol and

. glycerin to the bleaching solution will help to prevent penetra-tion to the lower layers. The actual emulsion used can be madeto assist in this direction also; thus the lower emulsion, firstcoated, may be made with a gelatin having a low water absorp-


tion coefficient, which is in turn hardened to the maximum ex-tent, while the upper emulsion is made with a soft gelatin whichswells readily and to which a small proportion of sorbitol isa.dded. It should. be pointed out, however, that ~nly compara-tively crude expenments can be envisaged with monopacks with-out refined facilities for both coating and processing.

Although it is beyond the scope of this book to enter into.the manufacture of emulsions containing color formers, the fol-lowing formulas for producing colored images by direct develop-ment of silver bromide emulsions may be given. A basic de-veloper is used as follows:

Sodium carbonate .Sodium sulphite .2-amino-$-diethyl aminotoluene hydro-

chloride .2 per cent potassium bromide .Water .

The blue-green coupler is:

2 -4-dichloro-r -naph thol .Acetone .Basic developer .

The magenta coupler is:

p-nitrophenylacetonitrile .Acetone .Basic developer .

The yellow coupler is:

Aceto acetanilide .Acetone .Basic developer .

40 g20 g

/C·5 g

30 ccrooo cc

g

5 cc250 cc

C.c5 g5 cc

0·5 g5 cc

2$0 cc

The image is formed in color on development with the abovesolutions, and is afterwards fixed in plain hypo solution. Thereduced silver is then removed by treatment with hypo-

COLOR-SENSITIVE EMULSIO S • 127ferricyanide when the clear dye images are left. In triple-coatedfilm it is conceivable that the dye couplers could be incorporatedin the respective emulsions, the one basic developer then pro-ducing simultaneously the three colored images. In the forma-tion of such a monopack it would facilitate production if a verythin layer of 3 to 4 f.I. of gelatin were coated between each emul-sion layer as is done in the case of Kodachrome film.

Chapter References

1. E. J. Wall, Photographic Emulsions.2. Idem.3. F. F. Renwick and O. Bloch, E. P. I33/769 and r33/700 of I920.4. Photographic Emulsions, p. 6$.$. C. E. K. Mees, U.S.P. 2,075,046.6. Dieterle and Reister, Zeit. [iir Wiss. Phot., 36,68-72, I937·7. W. Clark, Photography in the Infrared.8. Photographic Emulsions, p. 83; O. Magerstaclt, B. pat. $,932/I893,

and C. E. Pettit, B. pat. 8,956/r893·9. Brit. J. Phot., 55, Col. Su.p., 2, 6r, I908.

10. D.S.P. r,5r6,828.TT. B. Gaspar, E.P. 521,961.12. K. and L. Schinzcl, Das Lichibild, Sept. r936.

---------

CHAPTER VIIX-RAY A ID ULTRAVIOLET

X-ray Emulsions - Intensifying Screens - Lippmann Emulsions - Ul-tra violet Plates.

IN,.. the fir~t few years following. ~oentgen'~ discovery of theX-rays, It was found by empirical expenrnent that plates

coated with a slow silver bromide emulsion containing the leastpo.ssible quantit~. of s~lver iodide, gave the fastest response.HIgh speed to visible light had very little to do with the mat-ter. It must be remembered that at this time X-rays wereproduced with very crude tubes, excited by Ruhrnkorff coils~tt~d with the old hammer type of interrupter, capable of giv-mg a spark in air of only a few inches. Exposures of as muchas thirty minutes were quite common for thick parts of thebody, and it was not at first understood that short-wave radia-tion had a greater penetrative power, without which the longerwaves could not produce a result, no matter what quan.tity ofenergy was used. Fig. 35 shows how the penetrative power in-creases with decrease in wavelength in the case of aluminum 1

While the action of X-rays on photographic emulsion filmsis very similar to that of visible light, examination of micro-scopic sections through the developed film shows that the raysproduce an equal distribution of grains of reduced silverthro~ghout its whole thickness. It is thus evident that greaterdenslt~ ~an ~e obtained by increasing the thickness of the layerof sensitive SIlver salts, or alternatively its silver halide content.As stated, however, X-rays of different frequencies do not havean equal quantitative effect, and while the shorter wavelengths

X-RAY A D ULTRAVIOLET 129are needed for penetration of deep or thick parts of the body orfor metallographic work, the shorter or u soft" rays produce thegreatest blackening. For study of the sensitornetry of X-rayphotography the reader is referred to the work of Wilsey andPritchard."

In medical practice, the peak voltage used, which is a directmeasure of the penetrative power, varies between 35 and 90

.40\

1\\

1\\

1\I

'0 60 80 I

.36VJ~.32Q~.2BV)

~24

" .20

~\!l.16<:~.12

~~.08~

.04

00%.0

FIG. 35

kilovolts. This is ordinarily obtained from a transformer, andafter rectification is applied to the tube. The modern X-raytube has a cathode which is heated by a local circuit, and as theheat is increased, so the electrons emitted by it lower the resist-ance of the path between it and the anode, and so control thehardness of the radiation generated at the anode surface. Overthis range the one type of X-ray emulsion functions so satis-factorily that different emulsions have never been made forwork in different parts of the X-ray spectrum.

According to Charlesby," the response of a photographicemulsion to incident X-radiation is appreciably affected by the


absorption of the beam within the film layer, a fact that mustbe taken into consideration in X-ray sensitometry. This pointis mentioned, inasmuch as X-ray plates are used to some ex-tent in the measurement of radiation, which is d erminedby their photographic blackening powers under controlled de-velopment.

During the first years of X-ray photography, numerous sub-stitutes for silver bromide were investigated, the trend ofthought being that heavy salts of silver (such as silver tungstate,~or example), or perhaps salts of heavy metals themselves, hav-mg a high molecular weight, would absorb more of the radia-tion and convert it into chemical energy. An alternativethought was that the addition of heavy, inert metallic salts in-corporated in the emulsion might add to the effect of the ex-posure by secondary or scattered radiation. The results ofcountless experiments in these directions ended in a verdict forstraight silver bromide; but as the rays were only partially ab-~Ol'bed by their passage through an ordinary emulsion layer,It became obvious that the plates should be coated as richly aspossible. Hence coating weight was increased from, say, 120

milligrams per square decimeter to 180 or more millizrams in_ b ,

some cases by giving a double coating. Too thick a film hashowever, many disadvantages. '

Plates of excellent service were produced along these linesreducing exposure to a fraction of what had previously beennecessary, and at the same time immense advance was made inthe construction of apparatus for producing the high tensionsupply and in the tubes themselves. A very important advancewas next made by the introduction of so-called intensifyingscreens. A piece of card or celluloid coated with a layer ofartificial Scheelite (calcium tungstate), glows a brilliant blueto bluish-white under the influence of X-rays, and the fluores-cence excited provided photo-actinic light about five times as

\

X-RAY AND ULTRAVIOLET 13I

effective on a silver bromide emulsion as the original rays.Hence by placing the active surface of such a screen in closecontact with the emulsion film during exposure, the latter wasso greatly reduced that the great bulk of radiographic workquickly came to be done with intensifying screens. In so do-

. ing, the problem of the emulsion maker was somewhat changedin complexion, for now emulsions were needed which were suf-ficiently sensitive to X-rays themselves, but had also an op-timum response to the blue rays fluoresced by calcium tungstate.

The fact that only a low proportion of the incident rays isabsorbed byboth intensifying screen and emulsion, led to theidea of coating film base with emulsion on both sides, when twointensifying screens could be used, one on, either side of thedouble-coated film. An English patent was applied for by theauthor in I9I6 for the use of two intensifying screens in thismanner with thin glass plates (X-ray film had barely made itsappearance at this time), but the diffusion of the image on thefar side owing to its separation by the glass, led to its abandon-ment. Later, a double-coated film of magnificent quality, madeby the Eastman Kodak Company, overcame this problem ofdefinition, and double-screen technique became standard prac-tice, except where the finest possible definition was required, orfor" thin" subjects such as hands, wrists, etc. The grain ofthe calcium tungstate crystals not only tends to give a slightgranularity to the image, but each tiny crystal acts as a sourceof secondary radiation which throws off rays of the same wave-length as the original beam in all directions, and so causes dif-fusion. The general use of intensifying screens natura.lly ledto some attempts to modify the emulsions, as at least eightyper cent of the exposure reaction is probably that obtained fromvisible (fluorescent) light. At one time plates were even spe-cially manufactured having a maximum response to that partof the spectrum and having a normal coating weight. Present-


day emulsions, however.. are composed chiefly of silver bro-mide and can be used equally well with or without screens.

Yet another change is now affecting the position. Inasmuchas a full exposure for a deep part of the body in)(plves grossoverexposure of the soft tissues, detail in the latter is neces-sarily lost. To overcome tills drawback, a plain X-ray film isbeing used in many cases on top of the sandwich of intensifyingscreen - double-coated film -lower intensifying screen, sothat it receives relatively only about one-tenth of the effectiveexposure of the double-coated film. The un-intensified filmthus gives the detail in the soft parts, while the screened filmis correctly expo ed for the main subject. A method was pat-ented by Dr. Leonard Levy and -the author 4 for incorporat-ing the calcium tungstate in cellulose acetate base when castingthe latter, so that the film would be automatically self-screening.

A general emulsion for X-ray work, providing the necessarydensity and gradation, and suitable for use with or withoutscreens, may be made as follows:

A. Water .Nelson's X-opaque gelatin , .Ammonium bromide .Potassium iodide .

3000 cc360 g520 g

6 g

B. Silver nitrate 600 gWater . . . . . . . . . . . . . . . 150 ccStronger ammonia water to redissolve

C. Silver nitrate .Water .10 per cent nitric acid .

200 g800 cc

10 cc

Weigh out the salts and gelatin in A and allow them to standin the water for four hours. Raise to r ro? F. (43.30 C.) andcool back to 900 F. (320 C.). Add B rapidly from a jug atroom temperature with gentle stirring. After an interval ofthree minutes, add C at 1200 F. (490 C.) through a four milli-

I

\

. X-RAY A D ULTRAVIOLET I33

meter nozzle, with rapid stirring. Then immediately add 800grams of American Agricultural Co.'s high viscosity gelatin,and stir until dissolved. The crock is stirred while pouring inthe gelatin powder. Take to roso F. (40.50 C.). Then setquickly in cold small crocks in ice water. Wash after twenty-four hours until free from ammonia, then digest from one totwo hours at 1200 F. (490 C.).

Final addi tions are:2 per cent solution of phenol in spirit .5 per cent chrome alum solution .

ro per cent potassium bromide solution .

600 cc50 cc

roo cc

Total bulk is I2,000 cc. Recommended coating weight, 200to 220 milligrams per square decigram.

A practical reason for keeping the iodide low is that in fixingtanks such as are generally used, a high percentage of silveriodide will accumulate in the hypo solution and will convertpart of the silver bromide in films subsequently immersed intosilver iodide, which takes very much longer than silver bromideto fix.

Before leaving the subject of X-ray emulsions, it should bestated that investigations in recent years have indicated thatfluorescent screens glowing a color other than that of calciumtungstate, for example green or orange, might be usefully em-ployed in conjunction with a highly color-sensitized film. Azinc sulphide screen, for example, fluoresces bright green orbluish-green according to the method of manufacture, and withan orthochromatic emulsion will admit of excessively short ex-posures. Levy and West 5 have done much work in this di-rection, and have evolved a special screen for this technique.Whereas the fluorescence spectrum of calcium tungstate extendsroughly from 370 to 470 mu, that of their" fluorazure " screen

. is compressed into a sharply defined spectral band extendingfrom 390 to 490 mu.


ULTRAVIOLETSENSITIVEEMULSIONS.- While silver grainsare sensitive to light of practically all wavelengths shorter thanthe visible violet, the ordinary emulsion shows a distinct slacken-ing off in response at 280 I?/A. This is primarily due to thenatural opacity of gelatin to the ultraviolet. The response ofthe emulsion decreases steadily as wavelength decreases, show-ing a marked drop at 240 mu, and ending altogether at about200 mu, Some attempts have been made to sensitize emulsionsto ultraviolet by the addition of a sufficiently concentrated solu-tion of sodium salicylate (Tien-Kin). Application is also madeto the surface of the emulsion of oil or vaseline., which absorbsultraviolet energy and converts part of it into fluorescent lightof longer and more photographically active wavelengths. Dim-inution of the gelatin vehicle, with a consequent exposure ofthe silver bromide filtered with a minimum of colloid, still re-mains the best practice, as originally suggested by Schumann,"whose name is usually associated with this type of plate.

Schumann's technique is as follows: a plain silver emulsionis prepared, rather weak in gelatin. A five per cent concentra-tion of gelatin is suitable. The quality of the gelatin is statedto play an important part, and Nelson's o. I is particularlyrecommended. The emulsion is ripened and digested ,at oneoperation by heating to 1400 F. (600 C.) for a sufficient time,and is then set in ice water and washed. Very pure ingredientsshould be selected and, as a small trace of organic matter ap-pears to be prejudicial, freshly distilled water should be usedthroughout. By keeping the washing water cold (420 to450 F.), swelling of the gelatin will be minimized. The washedemulsion is next diluted with a large volume of hot distilledwater, breaking it down to a fraction of the usual concentration,and this very dilute emulsion is poured into a scrupulously cleantank on the bottom of which has been placed the glass plate thatis to be coated.

X-RAY A D ULTRAVIOLET 135

The temperature should not be above 650 F. for this sedi-mentation operation, otherwise there will be a tendency towardshalation. The silver bromide, with just enough associated gel-atin to make it adhere to the glass in a good continuous film,deposits gradually, and when the coating is sufficient the super-natant liquor is very carefully drained off and the plate allowedto dry in a flat position. Using a fluorite prism, and platesprepared in the above-indicated manner, the whole apparatusbeing in vacuo, Schumann is stated to have photographed thespectrum of hydrogen to 120 mu."

The individual worker will find by experience the best pro-cedure with his own working conditions and apparatus. Platesso prepared are insensitive to light of longer wavelength thanabout 300 mu, While Schumann worked largely with apyrogallol-soda developer, Lyman 8 recommends an ortol-potash formula. Any free bromide in the developer appears(0 increase the grain size of the image, and for that reason shouldhe avoided.

An alternative method of treating plates coated with a slowor medium speed emulsion such as an X-ray type has beengiven by Duclaux and janet." The plates are placed horizon-tally in a dish or tank filled'with ten per cent (by volume) ofroncentrated sulphuric acid in water, and are kept for four hoursat about 770 F. They are then carefully removed and washedill a very gentle current of water, and then dried. The treat-mcnt has the effect of removing both gelatin and silver, exceptfor a very thin layer. This is stated to give plates two hundredIime faster than commercial Schumann plates.

L1PPMANNTYPE EMULSIONS.- This type of emulsion yields!I color-sensitive, fine-grained plate suitable for experimentswith the well-known Lippmann process of natural-color photog-rnphy. The process depends on the interference of light wavesIraver ing the emulsion before and after reflection from the

I36 PHOTOGRAPHIC EMULSIO TECH· IQUE

back of the plate, which is in contact with a mercury mirror.It thus necessitates an exceedingly fine grain; Lippmann emul-sions are usually referred to, in fact, as grainless. A methoddescribed by Lehmann is as follows:

A solu tion of gelatin is first prepared:

A. Hard gelatin .Water .

20 g390 cc

This is dissolved and filtered through spun glass (glass wool)or cotton batting (wadding) and used at 950 F. (350 C.).

To 80 cc of the A solution add

Distilled water 10 ccSilver nitrate 4 g

To the balance of solution A, add and dissolve 3.2 g of potas-sium bromide. Taking care that there is no froth on this, itis slowly poured into the gelatin-silver solution at the sametemperature, stirring fairly gently during the precipitation andfor three and one-half minutes afterwards.

The following sensitizing dyes are then added immediately:

Pinacyanol (I: 1000 alcoholic solution) 4 ccOrthochrom T (r : 1000 alcoholic solution) 4 ccAcridin Orange (I :500 alcoholic solution) 4 cc

This mixture of dyes must have been previously warmed to860 F. (300 C.). It is added slowly with gentle stirring, notmore than forty-five seconds being taken. Under no circum-stances must the emulsion be subjected to any further heating,the object being to avoid grain growth and fog formation.

A very thin coating is applied to glass which has been so per-fectly cleaned that, if breathed upon, the condensation is evenall over and clears quickly. The coated plates are laid on acold slab to set, and are then washed in a trough with gentlyrunning water. Ten minutes should suffice to remove the potas-

/

X-RAY AND ULTRAVIOLET I37

sium nitrate. The washed plates are best dried in a horizontalposition; this is stated to give the most brilliant colors.

Chapter References

I. G. L. Clark, Applied X-rays, p. 25.2. Wilsey and Pritchard, J. Opt. Soc., I2, 661, 1926.3. A. Charlesby, Proc. Phys. Soc., 55,2, 0.293,1940.4. L. A. Levy and T. T. Baker, J. Roentgen Soc., I7, 1921.

5. Levy and West, Brit. Tourn: of Radiology, 6, 62, 1933.6. Schumann, Ann.der Pliysik , 349, 1901.7. ElIis and Wells, Chemical Action of U. V. rays, p. 14.H. Lyrnan, Photography of the extreme U. V., and ed., p. 60.9. Duclaux and j anet, J. de Physique, 2, 156, 1921.

CHAPTER VIIICOATING EMULSIONS ON GLASS

Preparation and Cleaning of the Glass - Substratums - Drying Cup-boards and Drying Problems - Coating Heads - Ventilation and Heating

THE c.oating of negative and positive emulsions on glass, atone time the only support available, suffered a severe set-

back when rollfilm, and later cut film, came into such generaluse. Plate coating has nevertheless been continuously main-tained on a large scale, particularly for the South American mar-kets, and in recent years certain advantages over film base pos-sessed by glass, especially in the photo-mechanical industries,have brought about a decided return to the dry plate. The studyof silver halide emulsions necessarily involves coating and testing.them upon a suitable base, and for laboratory and investigationwork glass is definitely, except where matters specifically re-lating to film technique are involved, the most satisfactory me-diu,m, especially as it lends itself so well to hand coating.

Carefully filtered emulsion of suitable viscosity is coated uponsheets of chemically cleaned glass. A known quantity is pouredon and distributed evenly over the surface while warm, anyexcess over the desired amount having been previously pouredoff· The coated glass is laid on a cold levelling table, wherethe emulsion sets to a firm jelly within thirty to sixty seconds.The set plates are placed in a dust-free dark cupboard to dry,where they may remain for from four to twelve hours, accord-ing to the temperature and humidity of the cupboard and ef-ficiency of the ventilation.

In commercial practice, the glass sheets are washed by a ma-chine in which they are made to pass between reciprocating

COATI G EMULSIONS ON GLASS 139

brushes and rollers. A cleansing solution such as soda is sprayedon them during their passage through the first part of the ma-chine. They next pass through water sprays, then betweenrubber rollers which squeeze off the superficial water, and arefinally sprayed with a weak solution of chrome alum or sodiumilicate, which leaves the surface with a substratum or " tooth"

which serves to anchor the emulsion to the glass surface andto prevent the danger of frilling or blistering during the proc-essing of the plates.

The chief commercial sizes of plates in use in the UnitedStates are 4 by' 5, 5 by 7, 8 by 10, II by 14 and 14 by 17 inches.Larger sizes are used in photo-engraving work and commercialphotography. Four by five plates are coated as 8 by 10

and cut, when dry, into quarters; 5 by 7 would be coatedas 7 by 10 and cut into two. The most popular English sizesare 3~ by 2!. (the higher figure is quoted first), 4% by 3t(quarter-plates), 6~ by 4* (half-plates), 8.z by 6~ (whole-plates), 10 by 8, 12 by 10 and 15 by 12 inches. The gauge ofthe glass increases -with the larger sizes; the sheets themselvesrun about one-sixteenth inch short of the nominal size in eitherdirection to insure easy fitting in plateholders.

The sheets of glass are delivered in crates, and require acertain amount' of sorting. The glass must be fairly fiat, uni-form in thickness, and. as colorless as possible. It must alsobe free from bubbles and flaws. Drawn glass, which has comeinto use recently, may have slight channelings, the worst ofwhich must be rejected, but" fiat" negative glass is ordinarilyslightly curved, and the concave side is placed uppermost onthe coating machine. The plates, after washing and drying,mu t be inspected to make sure they are all concave side toone direction so that no delay is caused in feeding them to themachine.

It is never advisable to use old negative glass from which the


developed film has been removed. The original image is liableto appear as a " ghost" in the newly exposed picture, and isprobably the explanation of the only genuine" spirit" photo-graphs which have puzzled laymen. Clerc states 1 that it maybe supposed that the presence of ultra-microscopic particles,probably of metallic silver in solid solution in the glass, are thecause. Suitable glass in small quantities, known as negativeglass, can be obtained for experimental purposes from suppliersof photo-engraving materials, such as Geo. Murphy, Inc., of

ew York, etc., or of course from any firm of glass merchants.A solution of potassium dichromate of two ounces to the pint,

to which two ounces of sulphuric acid has been very cautiouslyadded slowly, with stirring, is very useful for cleaning smallquantities of glass for experimental work. It may be appliedwith an old, flat nailbrush, but rubber gloves should be worn andcare taken not to get any splashes on the face or skin, as thechromic acid is very corrosive. The glasses must be verythoroughly washed under the tap afterwards and put into aclean plate-rack to dry; this rack should be kept entirely forthis purpose: Another method is to brush them with a hot twoper cent solution of caustic soda, afterwards washing thoroughlyunder the tap. Substratuming is done by dipping them into adish containing a two per cent solution of chrome alum afterthe final rinse and racking them without further rinsing. Theyshould be dried in a perfectly dust-free cupboard.

As a great deal of the success with test emulsions dependson the efficacy of the drying cupboard, this should next be de-scribed. Dust and bacteria are arch enemies of the plate maker.Mold spores are invariably carried on dust particles. A speckof dust dropped on to the gelatin-coated glass may deposit aspore which finds an ideal medium for growth, especially ifthe plates are dried slowly in a somewhat humid warm atmos-phere. The gelatin itself, in the process of manufacture, may

!

COATING EMULSIONS ON GLASS

pick up bacteria which remain latent and will ultimately infectthe 'emulsion. Certain types of bacteria develop below theurface (anaerobic types), and cause the formation of tiny lique-

fied local spots. Micro-organisms in some cases produce al-kaline growths, in other cases they are acid. The acid growthswill desensitize the emulsion and usually produce white spotsin a negative. The alkaline growths tend to produce black

A~==============~B

LL

FIG. 36

spots. There is plenty of dust in addition which carries nobacterial infection, but which, if it settles on the film while theplates are drying, may produce spots of one kind or another,or alternatively may cause scratching of the emulsion if the


plates are packed film to film. The usual procedure, of course,is to pack plates with thin card edge-separators between eachpair of plates.

A diagram of an easily constructed smallcupboard is shownin Fig. 36. If made 36 inches wide inside, 36 inches high, and

FIG. 37

12 to 14 inches deep it will accommodate a couple of dozen platesor more, S by 7 or 8 by 10 inch size, in two racks as indicated,The racks are made movable, resting on the supports SS asshown in the figure. If experimenting with papers, several stripsof coated paper can be suspended from the crossbar AB. Eachrack will require two I by I!-inch wood bars, their length beingjust under the width of the cupboard, attached to two crosspieces as indicated in Fig. 37, so that the complete rack can

FIG. 38

be laid on the upports. The rack bars must be notched at equalintervals as shown in Fig. 38, to support the plates more or lessvertically. Ventilation, top and bottom, must be fully light-trapped. An entry at bottom and exit at top can be built up

!

COATI G EMULSIONS ON GLASS 143

with baffles in any simple way with a little ingenuity. TwoIs-watt Mazda lamps, LL, completely concealed by invertedcans with baffled ventilation holes, will provide ample heat fora cupboard of this size, the hot air rising and causing some cir-culation. The temperature should not rise above 90° F. Thecupboard can equally well be mounted on the wall an inch ortwo above a radiator, the inside lamps being then unnecessary,or a concealed heater such as an electric flatiron, used in serieswith a resistance to reduce the heat, may be adopted. The heatin any case should not be switched on until the plates have beenin the cupboard for at least half an hour. A door is not shown inthe figure, but it is best made practically the entire size ofthe cupboard. It must of course be made thoroughly light-tight, and the whole of the inside should be painted with dead-black varnish.

The coating of the glasses requires to be' perfectly even, ofa carefully controlled thickness, and free from spots, bubbles,or blemishes. Emulsion is not, as a rule, used the day it isprepared, but i set off after making up and filtering, and re-melted for coating next day, with practically no stirring. Be-fore the coating, it is warmed to 105°-1 r o? F. and stood for

FIG. 39

from fifteen to thirty minutes. A convenient levelling tablecan be made with a sheet of plate glass three-eighths or one-halfinch thick, resting on three two-inch nickel screws (Fig. 39)·

I44 PHOTOGRAPHIC EMULSIO TECHN IQUE

The screws are set only about half an inch into the bench ortable, leaving an inch and a half above it. Care must be takenthat if two such screws are in line, the third must be placedsymmetrically at right angles to this line, so that the J.hree screwtops form the corners of an equilateral or isosceles triangle.

The plate glass is laid on top of the three screws and levelled.Setting a spirit level along a line between the two screws ABthese are adjusted until that side of the glass is dead level. Th~spirit level is then laid at right angles, that is, along the linefrom C to AB, and screw C is adjusted. Butterfly flanges mayvery usefully be soldered on to the under part of screws so thatthey can be turned without having to lift the glass plate everytime. A piece of I2 by I 6-inch glass is a useful size takin 0", '"four 5 by 7 or half a dozen 4 by 5 plates comfortably at onetime. The plate glass should be kept in a dish of cold water atabout 40° F. until just before coating, and the plates laid onit wet, without wiping the surface. For the sake of uniformwork it is recommended that the plate glass be cooled to aboutthe same temperature on every occasion. The glasses them-selves should be on the warm side, about 70° to 75° F. to assisteven flow of the emulsion. Elaborate setting devices made ofmetal with plate-glass tops, into which ice-cold water or warmwater can be run at will, are used by some, b~lt the simplemethod just described leaves little to be desired from the pointof view of results.

Keeping the size 5 by 7 inches as a good working unit fortest work, the volume of emulsion needed per plate is about tencubic centimeters. Transparency or lantern plates are bestcoated with six to eight cubic centimeters, but if this is founddifficult to distribute evenly, the emulsion may be diluted forcoating proportionally. A simple trick of getting the exactcoating weight is to use two little beakers as shown 'in Fig. 40.One, A, is clearly marked with a black stripe at a measured

/

COATING EMULSIO S ON GLASS

content of, say, 20 cc. The other, B, is similarly marked ata content of 10 cc. Beaker A is filled with emulsion at aboutreo? F. to the mark; to avoid bubbles, a piece of washed outcheesecloth or coarse muslin is laid over the top when pouringm. The whole lot, in this' case 20 cc, is poured quickly on to

I

A8

FIe. 40

the middle of the glass plate, which is balanced on the tips ofthe thumb and fingers of one hand. Quickly, but gently, theglass is tilted so that the emulsion runs to one corner furthestfrom the coater. It is then tilted in the other direction untilit runs to the opposite' far corner. By further tilting, it is madeto run to the opposite near corner, then over to the other nearcorner. It is given a deft twirling movement to even out theemulsion, and then sufficient emulsion is poured off the glassinto beaker B to fill it up to the IQ cc mark. The volumeof the emulsion left on the plate is obviously the difference be-tween 20 and IQ cc, or the amount required for the area in ques-tion. If the glass is not cold, and the emulsion is of the righttemperature and viscosity, it will remain perfectly fluid duringthis operation and will level out nicely. The coated plate isthen immediately taken by the other hand at one edge - thumbat the edge and fingers below the glass - and laid on the coldlevelling table, and left there until set.

A little practice will make coating come quite easily, and itwill be found that plates up to 12 by 15 inches can be coated


quite evenly in this way. But while it is somewhat extravagant,it is strongly recommended that preliminary coating experi-ments be made with emulsion in white light until the knackhas been acquired. The concave side of the glass is coated, andthis can be seen by looking along the edge of the g ass whenheld in a vertical position on a level with the eye.

FIG. 41

Setting depends, of course, upon the quality of gelatin used,its concentration, the amount of heat which has been applied,and the finals pre ent in the way of spirit, chrome alum, alcoholand so on. Certain proprietary chemicals are sold to reducesurface tension, and these affect coating to some extent. Itshould be contrived, from a purely mechanical point of view,that thirty seconds suffices to set the emulsion so that it canbe lifted from the levelling table and transferred to the dryingcupboard. Evenness of coating can be detected instantly bylooking through the coated plate at a low candle-power, clearMazda bulb. This should not be done with every plate, as itmay cause fog. Hand coating of panchromatic plates requiressome practice on account of the very low light in which it hasto be done, and the curious fact that perspective seems lost inthe dim green light of the panchromatic safelight. It is for-tunate, however, that panchromatic emulsions are far less sen-sitive when wet and that, provided direct rays are avoided, a

/


comfortable amount of light is permissible. If the tests onprocessing show fog, coating light should always be suspectedbefore chemical trouble.

In commercial coating, the glass, having been washed, dried,and stacked in a convenient wooden box (Fig. 41), with theconcave sides all one way, is placed on a table near the, coatingmachine, and slightly warmed to a temperature of about 75° F.The sheets are fed on to the machine, the operator wearing whitecotton gloves. The principle of the coating machine is indicatedin Fig. 42, where the glasses are fed on to a " band" of smalldiameter rollers, almost touching, which are seen revolving inaclockwise direction. As the plate is laid on the rollers, it iscarried forward until it passes under the coating head orspreader, indicated by S. It emerges coated with emulsion andpasses on to an endless band of felt or blanket running over

FIG. 42

rollers AB, the lower half of the band passing through a tankof cold water to keep it at a sufficiently low temperature toinsure setting by the time the coated plate has traveled to theend of the run. The set plates then pass on to a contiguousendless band running over rollers C, which are driven at aslightly faster speed. This causes the plates to become sepa-rated by intervals of about half an inch, so that they can beeasily picked up by the taker-off at the extreme end of the run.The plates are racked, and as each rack is filled it is carriedaway by another operator and placed in a section of the dryingcupboard.

148 PHOTOGRAPHIC EMULSIOl TECH IQUE

Many variations of this general scheme are naturally pos-sible. In place of the endless band of felt, for example, twoparallel metal chains running over a slate or plate-glass bedare frequently used. Chilled water runs along the bed, andthe plates actually ride along on the chains with a film ofwater in contact with the under side. In all cases, it is usualto make a tunnel of the part of the machine where the coatedplates are setting, with a light wooden or metal cover, in orderto make the travel dust-proof.

A matter that should not be passed over in connection withcommercial production, is that certain individuals appear tohave a bad effect on the emulsions, and it has not infrequentlyhappened that a man who may be a good coater cannot be em-ployed in this part of the factory. In spite of wearing cottongloves and although modern hygiene and ventilation, etc., wouldappear to make such a contingency ridiculous, personal contactand handling of emulsions makes it necessary to suspect andcheck up personnel when trouble from fog arises after all otherlikely causes have been properly investigated.

The greatest variation in coating machinery for plates isprobably to be found in the character of the spreader. Thesimplest possible form is that of a V-shaped trough made up oftwo pieces of flat glass, ground dead true at the bottom; theseare inclined to each other at an angle of about 40° by means oftwo wedge-shaped pieces of wood which also form the ends. Asindicated in Fig. 43, the bottom edges of the glasses do not quitemeet, but leave a parallel-sided slot of about a sixteenth of aninch, easily adjustable in putting the trough together, throughwhich the emulsion passes to the glass plates as they travel be-neath it. The four pieces - the two triangular ends and theglasses - can be taken apart and cleaned after each coating,and then reassembled by means of suitable clips to re-form thespreader. The complete V-trough is supported by uprights at

/


the sides of the machine, the height above the travel being ad-justed to the thickness of emulsion film required. The emul-sion is introduced into the trough from a tap leading from ajar standing above the machine, and is maintained at a constantlevel in the trough. In spite of the great simplicity of the V-spreader, in skilled hands it remains one of the most satisfac-tory coating appliances in practice.

FIG. 43

A more elaborate spreading device was invented by jamesCadett which consisted of a number of tiny silver pumps placed,in line, which delivered the emulsion from a number of pointsto the spreader proper, the length of throw of the pump pistonsregulating the volume of emulsion delivered. By controllingthis throw, and the rate of the endless band carrying the glass, .a very exact quantity of emulsion could be delivered. Theemulsion was run out from the pumps on to a silver plate ofslightly less width than that of the glass to be coated, and infront of it was a silver roller suspended on cords which almosttouched the surface of the plates and distributed the emulsionevenly over the whole width.

The cascade spreader is another type. Here the emulsionis delivered from a tap or set of pumps into a small trough

~------ - -- -------~

ISO PHOTOGRAPHIC EMULSION TECHNIQUE

formed by a strip of silver, AB (Fig. 44), attached to a smalltube of silver, T. When' the emulsion has filled the trough tothe height of the top of the tube, it flows over and falls down

·A

FIG. 44

the cascade, EF, which is another strip of silver sheet, the lowerend, F, of which comes below the tube, T, and under which theplates pass in the direction indicated by the arrow. Sometimesa thin strip or cc apron)) of linen or paper is attached to thebottom of the cascade to provide a more flexible cc lead on )) tothe glass surface. Here again some convenient type of con-stant level apparatus may be used to control the amount ofemulsion falling into the spreader, or it may be controlled bya series of Cadett pumps.

As an example of a commercial plate-coating machine 2 thefollowing details may be quoted:Feeding-on belt 4 ft., 6 in.Adjustable" fence" for guiding the glass in center of this belt I ft., 9 in.Endless belt under emulsion spreader, kept warm by warm

water in which the lower part of the belt travels 3 ft., 3 in.Washing rollers to clean under-side of the coated plates . . . .. I ft., 0 in.

/

COATING EMULSIO S ON GLASS

First endless belt for levelling out of emulsion, with separateunder tank of cold water 5 ft., 0 in.

First setting belt 7 ft., 0 in.Second setting belt 7 ft., 0 in.

These two are kept cool by water from one long tank below.Dry blanket belt 5 It., 0 in.

The total overall length of the machine is thus about 34 feet.The setting belts may be of cord, ribbon or flat gauze. Overthem is a nineteen-foot cooling tunnel provided with cold coilswhich cause cool air to fall on the surface of the plates.

A number of points must be taken care of in any form ofplate coating. It is hardly necessary to say that uneven dis-tribution is a grave fault, as is any form of streak or frictionline running along the length of the plates. Perfect mechanicalwork and adjustment is needed, and complete freedom fromvibration. A direct drive from a worm gear is almost sure togive trouble, and while a high speed motor may be geared downsuitably by a worm drive, the drive to the machine should beby belt. The proper control of viscosity, temperature and run-ning rate will have to be found by experiment for any machine,but the emulsion chemist should try to leave the coater as littleadjustment to make as possible. Thus all fast negative emul-sions should be capable of being coated at about the same speed,while medium slow negative and slow photo-mechanical ordiapositive types would form two other groups. The thicknessof the glass being usually less in plates which are to be cut tosmall sizes, some adjustment will be necessary in the height ofthe spreader or the throw of the pumps, etc., so that here againa whole day's coating of one particular brand or group of sizes isindicated for smooth running.

The modern dry plate must be so perfect to stand up to com-mercial requirements, that the greatest care is essential in thetreatment of the air of the coating and drying rooms. Many


firms -- such as Carrier ~ have specialized in air-conditioningfor the sensitized-products industry and will work out detailsfor specific plants, but they must be called in before and notafter the plans of a new plant are designed. As the humidityas well as the temperature must be carefully controlled, the airis usually taken from outside, after a preliminary filtering bya gauze that will remove gross mechanical dirt, and is pumpedthrough a metal chamber in which large numbers of finely atom-ized water sprays are so set as to give as perfect contact withthe air as possible. In an acid-laden atmosphere, the pH ofthe spray water may be kept slightly above 7, but ample wash-ing with normal water is soundest practice. The washed air,more or less saturated, is now passed over cooling coils for whichsome form of refrigeration will be required. The expulsion ofthe water in the cold chamber, which is provided with baffles toassist the process, naturally lowers the temperature, and thecold, but now dry, air is led through a heating chamber wherehot coils raise it to the temperature desired. Coating roomsmay be conveniently kept at 700 F. (210 C.). Drying cup-boards may be started at a few degrees above this, and gradu-ally warmed up to 900 F. (32 0 C.), the heat being turned offa short time before the racks are collected. The design of thedrying cupboards must be left to individual circumstances.They must be near to the coating machine, so that the racksof coated plates do not have to be carried too far. They shouldnot be too big -- a number of small cupboards capable of hold-ing 500 or 600 8 by ro-inch plates each, is better than a fewlarge ones. The racks are run on wooden rails in the cupboards,and both racks and rails must be of the smoothest possible char-acter to avoid friction of any kind.

Assuming that the dust problem is solved by the use of water-washed air, the next most serious problem is that of bubbles.A tiny bubble may get lodged in the spreader and cause lines on

COATIN G EMULSIONS ON GLASS • 153

a large number of plates unless it is quickly seen by the coaterand removed. Alcohol in the emulsion helps considerably inlowering the surface tension and reducing the tendency to bub-bles. A small atomizer filled with alcohol, or a mixture of al-cohol and acetone, is usually at hand to remove froth or bubbles.Careful filtering and skill in pouring and keeping the emulsiontranquil, are all helpful. Commercial specifics for lowering sur-face tension have been introduced in the trade and find usefulapplication. Dull spots, often known as grease spots, some-times appear on the surface a few seconds or minutes after coat-ing, and a saponifying agent such as an alcoholic solution ofsaponin may be helpful.

A very even distribution of the air in the drying cupboardsis essential, otherwise drying marks may appear which revealthemselves, on development, as local patches of uneven density.Any sudden change in the rate of flow of the air through thecupboards, will also cause drying marks or demarcation lineswhere the change took place. Some factories remove the rackswhen the plates are dry and place them in an after-d1'ying cup-board for a few hours without ventilation and at an elevatedtemperature, such as 1220 F. (500 C.). This is claimed 3 toimprove the keeping qualities and to be of especial value in thecase of plates destined for use in tropical climates.

Before cutting and packing, each plate is briefly examinedby looking through it at a safelight for bubbles, scratches orother defects. Glass-cutting machines are in general use fordealing with large sizes which have been coated for smaller ones.

Probably only those who have had to tackle the problem ofcoating plates year in and year out, are aware of the extraordi-nary diversity of faults which can develop in manufacture. Per-fectly good plates may be spoiled by the use of unsuitable pack-ing materials. ot only must the boxes be made of materialswhich have no deleterious effects -- and the glue used is as im-

--- ---------------~--~---~"~-=---------


portant as the card itself.- but the wrapping paper must bemost carefully tested for its effects, if any, by oven tests. Tendays at 1050 F. in an oven maintained at not above 65 per centrelative humidity is a fair test. The plates so "i cubated"should show no more than 0.02 extra fog density when comparedwith controls, and given normal development.· The carel separa-tors used for keeping each pair of plates from touching, or forgrouping each pair together, must also be made from card whichhas passed the laboratory test. Even outside labels require to befree from contaminating power, as also any slips or literaturewhich may be packed inside the boxes.. It is of course most important to keep a record of batch num-

bers, and a serial number given to the box, checking with theemulsion laboratory records, will enable complaints or faultsto be investigated.

Chapter References

1. L. P. Clerc, Photography, p. 157.2. T. L. Dixon and Co., Ltd., Letchworth, England.3· R. j ahr, Fabrikation der Trockenplatten, p. 236.

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CHAPTER IX

BROMIDE AND CHLORIDE PAPERSature of the Raw Paper - Paper Tests- Baryta Coating - Emulsion

Formulas - Laboratory Methods of Coating and Drying - CommercialCoating Machines - Drying Tunnels - on-stress Coating - Drying,Reeling and Packing

WHILE natural-color photography has re-introduced to alimited extent the transparency picture, which is viewed

by transmitted light or by projection, the natural desire in tak-ing a photograph is to have it finally in the 'form of a printwhich can be viewed by reflected light. Some form of print isindispensable in any case, if the original takes the form of anegative. A print on any photographic paper is of course alsoa negative, for printing a negative from a negative reverses theimage and produces the positive picture. Certain losses occurin the process, but modemsensitometry has gone a long waytowards reducing them to a negligible minimum.

One function of the print is to provide, in fact, some meansof control over the negative image and its possible shortcomings.If a negative is too hard, we can use a soft-working paper; ifthe negative be flat, we can print it on a vigorous paper, and soon. The most practical example of the application of this con-trol is seen in the work of the photo-finisher, who uses perhapsfour different grades of paper in making prints from customers'negatives, soft, medium, vigorous and contrasty. By selectinga paper for each negative merely by means of an experiencedeye, and taking each print out of the developing solution at theright moment, a remarkable uniformity of quality is obtainedin the prints from a range of negatives that, without such facil-ities, would be almost impossible to deal with.


One of the most important factors in the making of photo-graphic papers is the paper base. The production of a perma-nent image, provided the paper is handled in a proper manner,is a moral obligation on the part of the manufacturer, and withthis in view the testing of the raw paper becomes essential. Themanufacture of very pure, strong papers of close texture, dur-ability and uniform weight suitable for emulsion coating, is inthe hands of a comparatively few firms, and is a highly special-ized department of the art of paper making, of which the litera-ture is somewhat scant. Quality of water has in many casesbeen a deciding factor in the success of a photographic raw paperundertaking. The paper is ordinarily made in rolls 42 to 44inches wide, in' soo-meter lengths in the card thickness, and800- to rooo-meter lengths in the lighter weights. A squaremeter of light-weight paper will weigh 90 to 135 grams, cardthickness 210 to 270 grams, while various medium heavy weightvarieties come between the two.

The paper must show a uniform texture when examined bytransmitted light, and it must not stretch too freely when wet,nor shrink too much when dried after wetting. For this rea-son papers containing much wood pulp are unsuitable, and atest for wood fibers is usually one of the preliminaries in con-nection with an unknown sample. The following solution maybe used for the test:

Alcohol ,., ',.,',. roo ccPhloroglucin , ,.,., ,.,... 2 gHydrochloric acid , ', ' , 50 cc

This solution, freshly prepared, is of a weak yellow calor. Itsapplication to the paper sample will cause wood pulp or fibersto turn red. Another useful test is that suggested by Herz-berg; 1 a solution of twenty grams of zinc chloride in ten cubiccentimeters of water is mixed with five cubic centimeters of

BROMIDE Al D CHLORIDE PAPERS

water containing 2.1 grams of potassium iodide and 0.1 grams'of iodine. The solution is shaken up, allowed to stand, andthe clear portion decanted and used. A sample of paper whichhas been pulped by boiling with a five per cent solution of'caustic soda for a few minutes and well washed is treated withthis solution, which shows rag fibers as deep wine red, starchysubstances as blue, and wood fibers as yellow.

Impurities in the raw paper come into evidence most fre-quently in the form of small spots, which may develop in sizewhen the emulsioned material is kept for any length of timebefore use. Iron particles probably cause the most trouble.These can be checked by soaking a sample of the raw paper ina dilute acidified solution of the £erricyanide and Ierrocyanide ofpotassium for five or ten minutes and then washing with dis-tilled water. The presence of iron will cause a blue coloration;low-power microscope examination may be needed to detectsmall specks. The paper may otherwise be soaked in a oneper cent solution of pure nitric acid, dried, and then transferredto a dilute solution of potassium ferrocyanide; in this solutioncopper spots will be revealed as brown marks or tains, ironspots as blue. [eedless to say, photographic raw papers shouldbe practically free from loading.

Paper is used in the raw state only where a natural surfaceis required. Otherwise a coating of baryta is applied to, thesurface to be ernulsioned. A mixture of barium sulphate andgelatin, with a suitable hardening agent, is applied to the sur-face with reciprocating brushes. In a glossy paper about tenparts of medium hard gelatin would be used to one hundredparts of fine baryta; in a mat paper, the proportion of gelatinhas sometimes to be raised, on account of the heavier barytaused. Some kaolin may be included. An addition of glycerin,chrome alum (or formalin), and some citric acid usually, to-gether with a small amount of coloring matter, completes the


mixture. Red, yellow, or blue dyes are most popular, but thebaryta never looks a good color without some dye. A typicalbaryta coating machine 2 is seen in Fig. 45. The baryta mixture

FIG. 45. BARYTA COATING MACHINE

is applied to the paper, which after leaving the machine is runinto loops, the festoon traveling along a drying tunnel whichis maintained at a controlled temperature and humidity. Itis reeled at the end of the run, and is afterwards passed througha calendering process, where it is burnished and given thenecessary finish by its passage throuah highly polished roll-ers under considerable pressure. According to the baryta for-mula used and the calendering treatment, the urface producedmay be glossy, semi-glossy, half-mat, full mat, or rough mat.The baryta coating keeps the emulsion on the surface of the pa-per, so that the image lies superficially and is therefore more bril-liant. This incidentally makes the emulsion go much further.

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BROMIDE AND CHLORIDE PAPERS 159

Practically all development papers are given a non-stresscoating, to prevent abrasion or pressure on the emulsioned sur-face from causing black marks or streaks. Pressure will causethe film to become developable without exposure. The non-stress coating is a thin application of plain gelatin coated overthe actual emulsion, and in modern practice is applied as soon

N

FIG. 46

as the emulsion itself is thoroughly set. The method of apply-ing the two coatings on a double coating machine is seen dia-grammatically in Fig. 46. The mechanism for forming theloops is not shown. The raw paper from the roll R is led overa roller C which dips into a silvered trough kept filled with

•

160 PHOTOGRAPHIC EMULSIO T TECH IQUE

emulsion at constant level. As the paper is driven forward ittouches the emulsion and picks up a quantity which varies withthe rate of travel, which may be anything from twelve to fiftyfeet per minute. The paper, with its emulsion coaJing, is ledover a large-diameter cold metal drum, Dl' where setting takes

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FIG. 47

place, and it is then led over guide rollers to a second coatingtrough where the non-stress gelatin is applied by the roller N;this is set in turn by the second cold drum D2• Both coatingscan be applied by "kissing" rollers (Fig. 47), instead of bydipping. An advantage of the kissing roller is that it can bemade slightly shorter than the width of the paper, so that anuncoated selvedge is left at the two sides on which the leadingrollers over the setting drum can ride. In the case of dipping,the emulsion is necessarily applied to the entire width of the

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BROMIDE AND CHLORIDE PAPERS

FIG. 48

roll, and if clear edges are needed for leading rollers, a scrapingdevice must be attached to remove the emulsion for about halfan inch at the two sides.

By simple mechanical means, the coated band, thoroughlyset by the second cold drum D2' is picked up and formed intoloops eight or ten feet long, which depend from sticks that arecarried along by endless sprocket chains running along the topof the drying alley. The festoon thus travels slowly down the


alley or tunnel, which may be 200 or 300 feet long. Frequentlyit is made double, so that loops are carried on the sticks overa turntable at the far end of the tunnel and then travel up again,so that the paper is dry by the time it has reached.the reeling

FIG. 49

machine at the coating head end of the building. A doublecoating machine made by T. L. Dixon and Co., Ltd. is seen inFig. 48. _Tew rolls of paper can be attached to ends of fmishedrolls by means of auxiliary gear, so that coating can be continu-ous. In Fig. 49 is shown a reeling machine made by the samefirm; the paper is led in and out of the tension rollers to smoothout irregularities and creases before being wound on the drum.

With a little skill, a very good coating of paper can be ob-tained in the laboratory with just a dish of emulsion standingin an outer dish of warm water, laying a short, loosely rolled-

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BROMIDE A ID CHLORIDE PAPERS

up length of paper three or four feet in length on the surface,and pulling it vertically upwards with a quick, steady move-ment (Fig. 50). The roll of paper wiIllie on the surface and

----------FIG. So

as it unrolls the action will be substantially the same as thatof a dipping machine. If longer lengths are desired, a some-what elaborated arrangement can be used as shown in Fig. SI.This can be made with a little mechanical help, and indicatesa homemade paper-coating machine. A roll of twenty feet orso of raw paper is wound on to a wooden roller the spindle ofwhich is supported by two uprights, one on either side of thecoating dish. The dish is filled with emulsion just before coat-ing so that the bottom of the roll dips about one-quarter inchbelow the su'rface. The paper to be coated must be attachedto a paper" leader" which travels upwards to the roller R;which should be fixed as high up as is convenient. It thenpasses across to a second roller R, and down again to a fourthroller or drum on which the coated paper is to be wound. Asmall motor geared to this drum will drive the paper, and theleader will draw the coated paper over the upper rollers and

r64 PHOTOGRAPHIC EMULSION TECHNIQUE

down to the drum, when.of course coating must be stopped. Alength of about twenty-four feet can be coated at one time inthis way in an ordinary room, the length depending on theheight of the leading rollers above the dish and he distancebetween them. The motor should be geared to run the paperat ten or twelve feet per minute, and the drum should be beltdriven. The coated paper is left suspended, just as coated, overthe rollers until dry.

A bromide emulsion suitable for paper coating should givepure black tones, ample contrast, and a good scale of gradation.The manufacturer must of course supply different grades frorn >

soft to vigorous, with many different surfaces and weights, andmust arrange as far as possible that different brands of the same

FIG. 51

class have approximately even speeds. Actual speed is of littleconsequence in a bromide paper, except for enlarging purposesor automatic printing machines, and even then it is of an en-tirely lower order as compared with negative emulsions. Butit is nevertheless important to keep the speed of the different

BROMIDE AND CHLORIDE PAPERS

grades uniform, and this will be found to be by no means as easyas it might seem.

For laboratory or experimental work, the most importantthing to be done is to get hold of some raw paper, and here onemust depend on the courtesy of one of the manufacturers them-selves, who in general do not discourage amateur efforts. Somephotographers will wish to coat paper of their own choice inorder to secure some particular surface. Use pure rag paper, un-sized, such as Rive's or Whatman's. Thick papers are moredifficult to handle than thin ones, and may have to be sensitizedby floating on the emulsion one sheet at a time in an open dish.

When dealing with paper emulsions, useful information canbe obtained by trial coatings on glass. This will show if theemulsion is free from fog, which should not exceed 0.02 densityon normal coating, also if it has good latitude and has ampledensity-giving power. One hundred grams of silver halidewill coat eighty to one hundred feet of forty-two inch paper.

The reflection densities of a sensitometric strip depend somuch upon the paper surface and its absorption and diffusionqualities that a more practical indication of printing character-istics will be obtained from the coated paper. For information,it will be advisable to coat three or four strips say three feetlong by eight inches wide, trying to keep the speed of unrollingon the emulsion constant. Each strip may be coated at a dif-ferent temperature or a different speed to obtain the effect ofvarying coating weight. The faster the paper is unrolled, orthe lower the temperature of the emulsion, the thicker will bethe coating. Any form of strip sensitometer or gray step-wedgecan be used in an ordinary printing frame for making the tests,but results can be charted as characteristic curves only by use ofa reflection densitometer such as is described in Chapter XII.

It is all-important that the coating weight of a paper be cor-rect within reasonable limits. Thus, while negative emulsions


are usually made with forty to fifty grams of silver nitrate tothe liter, paper emulsions would be prepared with only fifteen totwenty-five grams per liter. This, of course, assumes that the Iactual volume of liquid emulsion applied to a given.area is of thesame order. One liter of emulsion should coat sixty to eightysquare feet of paper surface. The viscosity of the emulsion, the .surface of the paper, the coating temperature, and the speed ofthe machine will control coating weight. When dipping, theemulsion picked up is almost directly proportional to the periph-eral speed of the coating roller. Good blacks can be obtainedonly if the coating is sufficiently rich, but too heavy coating is-decidedly detrimental. It must be remembered that in the caseof a transparency, density and contrast can both be built up overa long range, but in the case of a development paper the max-imum contrast is attained very early during development, simplybecause the number of densities that can be distinguished by re-flected light from the maximum black is extremely limited. Thisdisadvantage renders it all the more important to limit coatingweight to giving in the highest exposures the maximum densitythat is desired for the particular brand, this maximum to bemeasured after normal development. Brilliance depends on thegloss of the surface, and the contrast and cleanness of the emul-sion, and can be largely lost if the latter sinks too much into thepaper owing to lack of viscosity or too slow setting.

A bromide paper formula by Trumm is as follows:

A. Water .Gelatin .Potassium bromide .Potassium iodide .

4000 cc350 g190 g

2·5 g

B. Distilled water .Silver nitrate .

2000 CC

2$0 g

C. Water .Gelatin .

2000 CC

600 g

BROMIDE AND CHLORIDE PAPERS 167The gelatin in A is allowed to swell in the solution of bromideand iodide for half an hour, and is then dissolved by heatingthe crock to 120

0 F. (490 C.). The silver solution, E, heatedto the same temperature, is added, pouring it through a funnelwith a fairly fine jet. A plain or separatory funnel can be usedfor the purpose, attaching to the end, by means of the shortestpossible piece of rubber tubing, a 1 or 2 inch length of barom-eter tubing about 4 mm bore. The funnel is mounted in a standabove the crock so that the jet is near to one edge, leaving ampleroom for stirring (Fig. 52). The emulsion so made is next

FIG. 52

further heated to 1400 F. (600 C.) on a water bath, and main-tained at this temperature for one hour. At the end of the hour,solution C, heated also to 1400 F., is added, and a further onehour's digestion is given. The emulsion is then poured out toset in a cold crock placed in ice water and stirred slowly until


gelling begins. It is then' broken up into noodles in the manneralready described for fast emulsions, and washed in about twelvechanges of five minutes each. The washed noodles are put intoa tared crock, remelted and made up, inclusive 0 finals, to aweight of 16,000 g. Finals are:

Fi:e. per cent chrome alum soluti~n 150 ccSPIrIt 1000 ccPhenol (dissolved in part of the spirit) 10 g

If a mat paper is required, 200 g of rice starch ground up inr ooo to 1500 cc of water is added and thoroughly stirred in,prior to filtering. When matting the emulsion, it is best to leaveout an equivalent quantity of water when making-up. Whenmade up to bulk, the emulsion is best set off and remelted nextday or when required for coating. An addition of an alcoholicsolution of saponin or extract of quillaia bark is frequentlyadded to give better coatings.

An alternative formula for a bromide paper emulsion madewith ammonia is given by E. J. Wall as follows. Three solu-tions are made up:

A. Water 1800 ccPotassium bromide 400 gCitric acid . . . . . . . . . . . . . . .. 400 gGelatin 150 g

B. Water 3750 ccSilver nitrate 500 g

C. Water (cold) 600 ccGelatin 150 g

Solution A, the gelatin dissolved, is cooled to 86°.p. (300 C.).To solution B is added sufficient stronger ammonia water toredissolve the precipitate. It is then added at the same tem-perature to A, through a jet, with continual stirring. The emul-sion is then poured on to C, which is cold, and the whole stirred

BROMIDE AND CHLORIDE PAPERS 169

and taken up to 1220 F. (500 C.). As soon as the gelatin inC has melted, the emulsion is set. It is then washed as usual ,the shreds melted and taken to 1040 F. (400 C.) and 450 g offresh gelatin is added which has been dissolved in 1600 cc ofwater. Final additions are 600 cc of alcohol and 30 cc of tenper cent chrome alum solution.

Unwashed emulsions have met with a limited amount of suc-cess for development papers. Inasmuch as too great an excessof free bromide would not only greatly retard development butcause the cc blacks" to have a decidedly greenish tint, it is im-portant that only a slight excess over the combining proportionshould be used. Also, on account of the hygroscopic nature ofammonium nitrate, the potassium salt would be used. An un-washed emulsion would be acid in character, and could be madeup somewhat as follows:

A. Potassium bromide .Potassium iodide .Potassium chloride .10 per cent hydrochloric acid .Gelatin .Water .

B. Silver nitrate .Water .

26 g

0·5 gg

5 cc30 g

560 cc

37·5 g300 cc

Heat both solutions to 1400 F. (600 C.), and add B to A througha fine jet with mechanical stirring. Digest for an hour and ahalf and then add So g dry gelatin, and stir for ten minutes untildissolved. Then add 50 cc of a two and a half per cent solu-tion of phenol in alcohol, and 5 cc of five per cent chrome alumsolution, and set off in ice water. Keep in cold room for threedays. Remelt and filter.

Formulas already given for transparency emulsions can withsome little adaptation be used for coating bromide paper. Pa-pers requiring a long scale of gradation may be made with di-


vided silvers, using a small amount of ammonia only and allow-ing a little ripening time. Mixed jet emulsions give excellentresults. In all cases, the choice of gelatin must be carefullymade; slow emulsions are probably more dependent. on gelatincharacter for their success than rapid ones.

The popularity of chloride papers, or " gaslight" papers asthey are known in the English market, is due to the extremeease with which they can be handled. The early papers of thistype were so vigorous that for a time specially soft negativeswere necessary to obtain the best results .. Studies of the be-havior of silver chloride, however, have made it possible to pro-duce chloride papers giving a wide range of contrast. A simpleemulsion suited to experimental work is as follows:

A. Sodium chloride. . . . . .. . . . . .. . . . .. .. 12 gGelatin 90 gWater 1000 cc

B. Silver nitrate 25 gWater IrO cc

Both solutions are heated to 1220 F. (500 C.), and B is addedto A either through a fme jet or in small quantities at a time- such as 10 cc. The liquid should be kept mechanicallystirred at a high speed. Between forty and fifty minutes' di-gestion is given at the same temperature, and the finals arethen added.

This is an unwashed emulsion, and as such is apt to attackthe baryta surface with the formation of minute black or whitespots. If the emulsion is washed, it may be used for coatingeither chloride paper or lantern plates. To the above quantityof emulsion an addition may be made of 5 to IQ cc of a ten percent solution of citric acid, the lesser amount tending to givebluish blacks and the greater amount greenish blacks. Three orfour cubic centimeters of a ten per cent chrome alum solution

BROMIDE AND CHLORIDE PAPERS I7I

and five per cent of the total volume of alcohol should be alsoadded. An emulsion of this type is an interesting one to ex-periment with, since all the work can be done in the subduedwhite light of the darkroom. Works-scale chloride emulsionsare bandIed in yeIlow light. .

The brilliance of chloride and chloro-bromide papers can beincreased by the discreet use of copper or thallium chlorides,although of most importance is the correct choice of gelatins.Small additions of citric acid, potassium citrate or sodium phos-phate may be made to washed emulsions to improve keepingquality, and formalin may be substituted for chrome alum inthe case of manufactured material which is destined to get somerough handling in photo-finishing shops. The amount of for-malin used would be about fifty to sixty cubic centimeters offorty per cent formaldehyde diluted with ten times its volume ofwater, for every kilogram of gelatin. Where emulsions are tobe washed, hydrochloric or citric acid is advisable in the mak-ing; hydrochloric would be added to the salts, but the citric acidmay be divided between salts and silver.

Intermediate between chloride and bromide development pa-pers there come, of course, the chloro-bromide variety, designedfor giving warm tones. The formula already given for chloro-bromide plates can be used with very little modification. Asa starting point for experiment, a mixture of equal parts of bro-mide and chloride might be taken. The silver halide concentra-tion should be about twenty-five to thirty grams per liter, andthe gelatin concentration about double this. The excess ofsoluble halide should be chloride, not bromide; the precipitationmade in the presence of free acid as indicated for chloride emul-sions; and after washing, only a trace of bromide should beused, otherwise the tones will tend towards a greenish hue. Ithas been the experience of some American emulsion chemiststhat halides of barium, cadmium and other metals give better


warm tones than those 0.£ potassium, sodium and ammonium.The inclusion in the emulsion of anything which will tend torestrain development may also help; the citrates and boratesare examples. But in both types of paper, chloridesand chloro-bromide, the manufacturer has to correlate his emulsion coat-ing with an optimum developer, the latter being essentiallyworked out for the material in question. Thus A's paper de-veloped with A's formula might work excellently, while usedwith B's formula the results might be very poor.

Care must be taken to avoid froth and bubbles on the surfaceof the emulsion in coating. For laboratory experiments, theemulsion should be filtered just before use, through a piece ofwetted cheesecloth, and poured down the side of the dish toavoid friction, holding the crock or beaker near to the dish.Large bubbles can be broken by the touch of a moist glass rod;froth can be dispersed by the cautious use of a little alcoholblown from an atomizer. Too much alcohol will cause the emul-sion to coagulate in the form of little strings or flecks whichmust be removed.

The non-stress coating applied to the emulsioned paper in adouble-coating machine, which is composed of gelatin, spirit,and some hardening agent, may suitably contain formalin,chrome alum being preferable in the sensitive emulsion. Thedouble-coated paper takes somewhat longer to dry than paperwhich is not non-stress coated. It is important, for example,to arrange the inlets of air in the drying tunnel so that warm,dry air is blown gently into the loops from the sides of thetunnel, and that sufficient air is directed on the bottom of theloops, where drying is apt to lag. The amount of air requiredto be pumped into the tunnel- and it should be pumped inunder slight pressure so as to form a plenum, and not merelyextracted - will naturally depend on the temperature and rel-ative humidity and on the amount of water to be evaporated

BROMIDE AND CHLORIDE PAPERS 173during the run from coating head to reeling machine. The in-clusion of a small quantity of Arlex may be made in the caseof continuous running, which acts as a plasticizer and is onlyslightly affected by changes in humidity; some coaters preferto run the festoon to the end of. the tunnel, and to leave thepaper for some hours in a 50 per cent relative humidity atmos-phere to get into equilibrium before reeling. This is only pos-sible where comparatively small coatings are made.

A new type of bromide paper has recently been introducedwhich is capable of giving images of either high or low contrastaccording to the color of the light used in printing (Defender" Varigarn " and Ilford "Multigraph"). These papers arecoated with a mixture of two different emulsions, one sensitiveto blue-violet only and having, say, a high gamma, the othersensitized to green by means of a suitable dye, and having alow gamma. The contrast obtained with the one paper canthus be varied at will by merely using the appropriate filtersover the printing light or the lens of the enlarger, thereby bring-ing into requisition the emulsion required.

Some causes of failure with slow development papers maybe given, in conclusion. Weak, grayish images may be due tothe emulsion taking up too much water in washing, and hencecausing a deficiency of silver. Low viscosity will give the sametrouble. Gray images with a tendency to fog are usually dueto over-digestion; the remedy is to reduce the time or the tem-perature of the cooking. An unsuitable gelatin may also be thecause. General fog may bedue to an unsuitable gelatin, too hightemperature for emulsification, insufficient excess of halide, im-pure chemicals, and, of course, too much exposure to light.Black fog indicates an excess of silver nitrate, due to incorrectweighing or miscalculation of a formula. Partial fog is some-times caused in a mixed-jet emulsion through the silver leadingthe salts and not vice versa. Innumerable tiny black specks on


development, or pepper, are due to coarse precipitates or aggre-gates of silver halide which, being insufficiently clothed withgelatin, are reduced without exposure. These rarely occur withan ammonia type of emulsion, but are very apt t creep intochloride emulsions, having a serious effect on the brilliancy ormaximum density. A cubic centimeter or two of the emulsionjust after it has been made, should be spread over the end ofa glass strip and examined by transmitted light, for example bylooking through it at a low power incandescent lamp. The ad-dition of the second gelatin dry instead of in solution, helps con-siderably to break up or disperse aggregates; finer nozzles forthe silver, better agitation during precipitation, or an increasein citric acid, will help to prevent pepper formation, or again achange of gelatin or its distribution between salts and silver.

Chapter References

1. W. Herzberg, Die Papierpriifung, I907.2. T. H. Dixon and Sons, Ltd., Letchworth, England.

CHAPTER X

FILMS, EGATIVE A D POSITIVETypes .of Ba e - Substratums - Substratuming Machines - NegativeFilm Emul ion - Po itivc Film Emulsions - Experimental Film Coat-ings on Roll and Cut Film - Film Coating Plant - the Drying Tunnel- Air-conditioning - Films for Imbibition Emulsions - Static - Com-mercial Defects - Packing

THE film bases on which sensitive emulsions may be coatedare of two kinds. One is celluloid or " nitrate" film, which

contains about 8S per cent of nitrocellulose and IS per cent ofcamphor, the two being dissolved in about five times their weightof a mixture of about 37.S per cent of 85 per cent alcohol and62.S per cent of ether, to form the dope from which the baseis cast. The other ba e is cellulose acetate film, which is prac-tically non-inflammable, and is compulsory for sub-standardmotion-picture film and is becoming used on a very large scale forgeneral photographic materials. Cellulose diacetate is madebyheating cellulose with zinc acetate solution and treating thecooled mass with acetyl chloride. It is dissolved chiefly in ace-tone, to which a plasticizer such as triphenyl phosphate, dibutylphthalate, etc., is added, together with tetrachlorethane, methylalcohol or benzene. The recent introduction of triacetate inplace of diacetate produces a tougher film, though the use ofa special substratum is indicated.' The mixed dope is filteredthrough filter presses, and is generally fed by gravity from anupper mixing room to the casting machines.

The dope is cast into the thin sheet used in film coating bytwo different methods. One of these is known as the big wheel

I76 PHOTOGRAPHIC EMULSION TECHNIQUE

system; the dope is distributed by means of a spreading deviceon to the flat periphery of a large highly polished metal wheelor drum which may be sixteen to twenty-five feet in diameter.The dr~m rotates at the rate of two or three feet-per minute,and as It revolves the dope solvent evaporates and leaves thefilm sufficiently dry towards the end of the revolution for it tobe parted from the metal surface and led into a drying chamber;here the remaining solvents are driven off, after which theband of film can be reeled. Much film base manufactured to-day is cast on an endless band type of machine. By this method,a band of copper about ninety feet long travels slowly over twolarge drums some three feet in diameter and spaced about fortyfeet apart; the whole is encased in a metal cover so that theevaporated solvent can be collected and condensed. Over thetop of one drum is the spreading device, from which the dopeis distributed on to the band. The band itself however is, ,usually coated with a skin of gelatin, glycerin and soap. By thetime the coated part of the band reaches the end of the run itis dry enough to be parted from the skin, and can then be passedthrough a final drying chamber and the residual solvent drivenoff. In the case of triacetate film the base is sometimes ziven

oa treatment 'before being finally dried, which acts as a sub-stratum and anchorage for the emulsion, thereby doing awaywith the need for the application of a substratum in the emul-sioning plant.

In the ordinary way, both celluloid and cellulose acetate re-quire a preliminary treatment to secure satisfactory adhesion ofthe emulsion to the base, a problem that has involved consider-able research to find a satisfactory solution. It will incidentallybe found one of the mo t troublesome parts of the technique inlaboratory coatings. The function of the substratum is to etchand attach itself to the film base on the one hand, and to" stick" to the gelatin of the emulsion on the other. The sub-

FILMS, NEGATIVE AND POSITIVE I77stratum most commonly used in early practice was thus a solu-tion of gelatin in a solvent such as acetic acid or salicylic acidbroken down with methyl alcohol. Ten cubic centimeters ofglacial acetic acid may be put in a test tube and powdered gel-atin added gradually, warming the tube gently over a Bunsenburner or alcohol lamp until no more gelatin will dissolve. Thisstock solution can be diluted with 200 cubic centimeters ofmethyl alcohol, to which two or three cubic centimeters of waterhas been added. This substratum works wel1 with cel1uloid,but if it is to be used for acetate film, a part of the alcoholshould be replaced by acetone. Five per cent of cel1osolve ace-tate may also be added.

For cut sheet film Wall 2 recommends the following;Pyroxylin "............................. 0.5 gGelatin 0.5 g

Glacial acetic acid 33 ccMethyl alcohol 1000 cc

The gelatin and pyroxylin are dissolved in the acid by heat,and the alcohol then added. The drawback to an acetic acidsubstratum is that it may lower the pH of the emulsion in con-tact with it and gradually slow it down, or affect color-sensitizingdyes. An alcoholic solution of caustic potash, to which a smallproportion of celluloid solution (celluloid clippings in amyl ace-tate) is added, is dangerous from the opposite standpoint. Someof the recently introduced compounds of the Carbide and Car-bon Chemical Corporation's research department have the ad-vantage of giving a " tooth" to the film by slightly etching thesurface through their mild solvent action. Cellosolve acetateis one example. The methyl compound is ethylene glycol mon-omethyl ether acetate, is miscible with alcohol, soluble in water,and soluble in cel1ulose acetate dissolved in acetone.

An organic partial solvent, suitably diluted, can be used togive a front etch for adhesion and a back etch for anti-curl


purposes. In some cases. the film base is first coated with a thinapplication of hardened gelatin to offset the curl imparted bythe emulsion. This balance is very important, as otherwise,during development in filmholders, cut film would <41rland dropout of the grooves, while rollfilm would be unmanageable after

FIG. 53

processing. Too deep an etch will of course give an opalescentappearance to the film, although this disappears if a coating ofgelatin or emulsion is applied afterwards.

Any form of substratum mustbe applied in a dust-proof alleyor chamber of carefully controlled temperature and humidity.The average application should dry in three to five minutes ata fairly high temperature such as 95° F., so that the roll of film

FILMS, NEGATIVE A D POSITIVE 179

base can be run continuously and reeled up at the end of themachine. It should be rolled up on large drums in order to makethe base lie flat when running through the emulsioning machine.A large inverted U'-shaped substratum machine much in use isconstructed by Koebig of Radebeul, Dresden, but a more recentform of apparatus is shown in Fig. 53, a compact subbingmachine made by T. H. Dixon of Letchworth, England.

In substratuming base for special purposes, a coloring mat-ter is sometimes introduced, as in the lavender motion-picturefilm of the Eastman Kodak Company, and the" gray base"film. The latter is claimed to give negatives of better quality,without interfering with gradation. The coloring matter, beingincorporated in the water-insoluble base material, will obviouslynot be removed in processing. While therefore some anti-halation dye may be used, it will be necessary to resort to adye in the back gelatin coating if such dye would interfere withthe printing qualities of the finished negative; dyes which canbe eliminated in an acid fixing bath etc., have been referredto on page 80.

Owing to dirt which inevitably creeps in during manufacture,transit, and general handling, the film base may be wiped orcleaned prior to subbing, by passing it through tightly coupledrollers or pads covered with suitable material. The cleaned andsubstratumed film base is coated at' a fairly rapid rate - any-thing up to forty feet per minute - and after the applicationof the emulsion, it is passed over a large copper drum filled withchilled water which sets it sufficiently for it to be carried overon rollers to a mechanical' festooning device which forms theband into loops depending from sticks as in the case of papercoating. The loops will be shorter and wider, owing to the na-ture of the base as compared with paper; loops five feet deepare customary in the heavier weights, and longer loops in thecase of 5/rooo and 3/1000 base. The festoon travels along the

180 PHOTOGRAPHIC EMULSIO J TECHNIQUE

drying alley or tunnel, which should be constructed with ceiling,walls, and floor, of concrete or similar material sufficiently sur-faced that when hosed down no dirt can remain on them; side

h h " "tgutters should be rounded off so t at t ere are no ~orners 0

trap dust. Needless to say, both temperature and hum~ditymust be controlled, and the circulating of the washed air soarranzed that the bottoms of the loops dry in the same time aso

FIG. 54. REELING MACHINE

the lengths. With a drying alley 300 feet long, that is 600 feetrun if the alley is double width with a turntable at the far end,the film should be easily dried by the time it reaches the end ofthe travel so that it can be reeled and the coating process thusmade continuous. A typical film reeling machine (Dixon) isseen in Fig. 54.

Coating small quantities of film in the laboratory or in thedarkroom has naturally to be done on an entirely different basis.For short coatings on an improvised machine, lengths of ten ortwelve feet of the 3/Iooo-inch thickness are recommended. If,

FILMS, NEGATIVE AND POSITIVE 181

however, cut film is wanted, the 5/1000 or 7-!/Iooo-inch basemay be cut into sheets on a clean print trimmer, and individualsheets squeegeed down on to " sticky backs " - glass plates pre-pared with a non-drying gelatin mixture. A small machine canbe made quite easily in the manner indicated in Fig. 55. A cou-

FIG. 55

pIe of IQ or I 2-inch rollers, turned true and polished in the lathe,of about 2-! inches diameter, are mounted on spindles runningin ball bearings fixed in the two uprights shown screwed to thebase. The rollers should be given two or three coats of shellacvarnish. The distance between the upper and lower roller, ofcourse, controls the length of film that can be coated, as it isfitted over the rollers in the form of an endless band. It is aconvenience if one of the rollers can be adjusted in the verticaldirection so that the band when fixed can be pulled taut. Whilefilm cement may be used for making the join, a piece of one-inch scotch tape will answer the purpose quite well. The lowerroller is the" coating roller," the dish of emulsion being broughtup to it so that the film dips into it about one-quarter inch; theemulsion dish should be stood in an outer dish of hot water.


While the little machine may be driven by means of a handleconnected by a belt to one or other of the rollers, it may equallywell be operated by a large driving wheel, belt-driven from a

FIG. 56

slow speed motor having a very small diameter driving wheel.The speed of running should be about twelve feet per minute,using a viscous emulsion at a temperature only a few degreesabove the setting point. In many commercial machines theband of film, after dipping, is run almost vertically upwardsthrough a rectangular-shaped cold shaft, fitted internally withbrine pipes, so that the surface of the emulsion is set by thetime it reaches the cooling drum above it. This drum, whichis usually of copper with cold water circulating through it, will

FILMS, NEGATIVE AND POSITIVE

cause trouble from condensation and consequent buckling ofthe film unless continually wiped with a crossbar covered withabsorbent material, or unless other equivalent steps are taken.While the pull of the drum may suffice to drive paper forward,film base requires some assistance, and either small weighted

FIG. 57

wheels may be fitted at the edges of the band, to ride on theselvedges, or a suction roller or belt may be applied immediatelybehind the drum. Suction rollers are used in many industriesand can be obtained from a number of American manufacturers.


The roller is perforated, and suction is applied through the hol-low shaft by means of it small motor-driven pump; the filmclings to the surface under the vacuum and the rotation of theroller pulls it forward and feeds it on to the festoon.

A small laboratory coating machine 3 is shown in Fig. 56,with which the film as it is coated is led by a leader of wastefilm or paper over the top rollers and down to the reeler. Anelaboration of the machine, as manufactured by Dixon, isshown in Fig. 57, where arrangements are provided for doublecoating. Double coating is used on an increasing scale, and hasmany advantages. By coating an extreme-speed emulsion ontop of a slower type, greater latitude is given and better buildingup of the image can be obtained, with increased densities inthe highlights. Each coating must necessarily be of a lowercoating weight than in the case of single coatings; halation islessened, and by dividing the characteristics between the twolayers, a film of improved keeping quality is produced.

Heavier base, the 7-t/rooo-inch, is the weight most easily-coated in the laboratory in small quantities. It may be cut intosheets of a convenient size, say eight by ten inches, and stuck,down on sheets of prepared glass, which are then dealt with ex-actly as dry plates. The celluloid or acetate base should be firstsubstratumed on the side which is to be coated. The" sticky-back" glass is made by coating it with such a solution as thefollowing:

Hard emulsion gelatin ., ,.Sugar syrup " , ,., , .Glycerin ., ,.,',.,', , .. , ,.,Ten per cent chrome alum solution , , , , , ..Water ., ,.,"", ,., ,', .. , ..

60 g60 g80 cc5 cc

800 cc

About forty grams of Arlex " may be substituted for theglycerin.

The gelatin is swollen in the water in the usual way, then

FILMS, NEGATIVE AND POSITIVE . 185

melted and brought to a temperature of 1220 F. (500 C.) andthe other ingredients added. Fifty to sixty cubic cen timeters ofthis mixture will coat a sheet of glass eight by ten inches, An-other formula for the sticky dope is as follows:

Hard emulsion gelatin .".".,"", .. , .A rlex "., ,., " , .Sugar syrup ,., "., , 'Dextrin , .. , ,Waler to make , .

75 g30 g60 g20 g

1000 cc

The gelatin is swollen in 700 cubic centimeters of the water andis then melted and the sugar and Arlex added. The dextrin ismixed into a smooth paste with water, then thinned down withthe balance of the water and heated until it forms a paste. Thisis mixed in with the gelatin solution, and if needed a little spiritcan be added. The filtered material in either case is coated onthe glass sheets in the manner of an emulsion. The plates shouldbe given ample time to set, three to five minutes being neces-sary, and they may require two or three days to "dry." Thelarge amount of glycerin or Arlex keeps the coating alwaystacky.

A sheet of sticky glass is taken, and a sheet of celluloid sub-,stratumed side up, is slightly curved into a U-shape, convex sidedownwards, and lowered until it touches the middle of the plate.The two ends are then gently lowered, and with a flat squeegeethe sheet of film is brought into firm contact with the tacky sur-face. The squeegee should be one made of black rubber. Awooden roller coated with billiard cloth or piano felt may beused instead.

In coating such film with emulsion, the inherent disinclina-tion of all film base to take kindly to gelatin makes its distribu-tion and adhesion difficult. Hence, while the plate is balancedon the finger-tips as has already been described in connectionwith dry-plate coating, it may be necessary to lead it over the

r86 PHOTOGRAPHIC EMULSIO TECHNIQUE

surface with a clean, warm, glass rod about a quarter inch indiameter held parallel to the glass, then carefully laying it onthe levelling slab. Owing to the heat-insulating effect of thefilm base and thick gelatin coating, setting will b slower thanin the case of a straight emulsion on glass, and this extra timemust be borne in mind.

The coated films are dried on the gla s in the drying cup-board in the ordinary way. When dry they are stripped off.Here a word of cau tion is necessary. One corner should beraised with a clean knife or spatula and taken between thethumb and finger. Holding the plate at the edges with the lefthand, on which a cotton glove should be worn, the free edge is ,taken between the thumb and finger of the right hand and thefilm gently but firmly pulled away from the glass, taking carealways that the plane of the film makes as acute an angle aspossible with the glass. The film must never be pulled up verti- ,I

cally or backwards, or it will show stress marks. Too rapidpeeling, or the use of any force, or any stop made during thestripping, may produce a static electric charge or a stress, whichwill appear as a fog mark on development. Buckles in thefilm show up on development, and are known as swallow marks.

The film should always be coated a little bigger than the sizerequired, so as to provide space for trimming. In trimming, itshould be remembered that cut film is about two millimetersless in width and length than the quoted commercial size. Thusin cutting a 4 by s-inch film, the size would be made about 3Hby 4H inches, otherwise it will not fit readily into the filmsheaths or holders. The cut sheets may be packed emulsionto emulsion, each pair being wrapped in its own black paperfolder. The black paper should be of approved quality andthoroughly dry.

Any emulsion may be used for film coating, but as already


indicated it should be of more viscous character than is neededfor plate coating. A suitable gelatin for film emulsion is quotedby Heyne as one which requires forty-five seconds when madeup as a two per cent solution to flow through the Ostwald vis-cometer. The same author suggests for a negative film emulsion

a formula which has been already' given on page 68. The prob-lem of keeping quality is somewhat more complicated in filmproduction, on account of the character of the base itself andthe substratum employed, and the alteration already mentionedas possible has been prevented to a large extent by the provisionof an inert hydrophylic colloid buffered at substantially thesame pH as the emulsion, or by treatment of the emulsion it-self. Details of the more rapid types of panchromatic emul-ion for film coating will be found in Chapter XIV.

A positive emulsion suitable for motion-picture work must

r88 PHOTOGRAPHIC EMULSIO TECHNIQUE

have a small, uniform grain and possess the necessary qualitiesfor both soun-d and picture reproduction. A fine-grained, long-scale emulsion is needed for both positive and negative soundrecords. While commercial positive gamma is usually in theneighborhood of two, it must be remembered that the soundis ordinarily restricted to a part of it only; negative gammaranges from 0.5 to 0.9, and the exposure is so regulated as tokeep the maximum value below the shoulder of the characteris-tic curve. Fig. 58 shows a smear of commercial positive motion-picture emulsion at X rooo magnification and indicates the ex-tremely uniform grain, which is nevertheless coupled with along straight-line portion of the curve.

A formula for a positive emulsion is as follows:

A. Gelatin .. - _ : _. . .. 400 gDistilled water 2500 cc

B. Potassium bromide. _. : __ _ __ 400 gPotassium iodide . - . - _ _. IS gWater .. __ _. __ .. 1500 cc

C. Silver nitrate '" - __ __ __ . . .. 500 gDistilled water . - _ . . .. 1500 cc

D. Gelatin _. - - - - - .. " _. __ " 350 gWater _. __ 1000 ccAmmonia (s.g. 0_910) .......••.......... 45 cc

This is a mixed-jet type, so that Band C are poured throughfive- to six-millimeter nozzles, at temperatures of 1400 F.(60

0 C.) and 770 F. (250 C.) respectively, into the crock con-

taining the emulsifying gelatin A, which should be at about120

0 F. (500 C.). The emulsion is digested at 1200 F. forthirty to forty-five minutes, and is then poured into D, whichhas been previously heated to the same temperature. The emul-sion may be further digested after washing if the tests show itto be necessary.

FILMS, N EGA TIVE AND POSITIVE

An all-ammonia positive film emulsion may be made as below:A. Water . . , , __

Ammoniu-m bromide , , , , . _. _Ammonium iodi le .. , . , , , , .. _, , .Soft gelatin .. , . , , , .

B_ Water ,. _, , . , . , . , , __Silver nitrate , .. , _.. _. , _, . , , , . , . , , , .. __Ammonia to redissolve.

1200 cc150 g

4 g120 g

100 cc200 g

Half the silver is poured at 770 F. through a nozzle or jet intothe crock containing the A solution (at 1050 F.); the other halfis then" flopped" in without stirring, and 200 grams of dry,high-viscosity gelatin is then added and stirred in until com-pletely dissolved. The emulsion is maintained at 1050 F. orjust under, then set in ice water. It is very thoroughly washedin order to get rid of all the ammonia, and made up with spiritand finals to 4000 cubic centimeters. Coating weight shouldbe 90 to lIO milligrams per square decimeter.

In view of the increasing interest in the making of color printsby the wash-off relief process, and by superimposition of threedyed gelatin images on film base, some description of the ma-terials used should be given. The original idea of using gelatinreliefs as matrices for producing the elements of a subtractivetransparency must be ascribed to ,E. Sanger Shepherd, to whomthe Progress Medal of the Royal Photographic Society wasawarded for this work nearly forty years ago.

A wash-off relief film can be used in two ways. One wayis by sensitizing a slow gelatine-bromide emulsion with dichro-mate, exposing it through the back of the film and developing itin hot water; the washed-ou t relief image is fixed and washed,when it can be used to imbibe the necessary dye, which can betransferred to paper or left in the film if the latter is to be madeone element of a three-color transparency. The other way is toexpose the film under a negative and after developing it to

I90 PHOTOGRAPHIC EMULSIO TECH JIQUE

harden the image with a 'bleaching bath, so that on treatmentwith hot water the exposed image alone remains, with its tonescale duly represented by graduated thickness of the gelatinrelief. This may be used as a matrix for taking ut' the neces-sary dyes, which can be transferred to a plain gelatin-coatedpaper previously treated with a mordanting solution.

A wash-off relief emulsion may be coated either on glass oron celluloid. Glass was used in the Calor Snapshots process,nitrate base is used in the Eastman Wash-Off Relief process.An orange dye is used in the latter film to prevent irradiationwithin the film and to keep the image in the layer nearest thebase (exposure is made through the base). Any slow bromideemulsion may be used for wash-off relief or imbibition films;but they must contain no hardening agent, and should be dyed'with about twenty-five cubic centimeters of I: 200 alcoholicsolution of naphthol orange per liter.

Such films may be sensitized with a three per cent solution ofpotassium dichromate, exposed under a negative through theback, and developed with hot water, then fixed in plain hypoand washed. Or develop in Eastman D-II, rinse well, andbleach for two to four minutes in a mixture of equal parts of:

A. Ammonium bichromate . . . . . . . . . . . . . . . . . 5 gSulphuric acid (conc.) ccWater 750 cc

B. Sodium chloride 11.5 gWater 750 cc

Then develop in water at 1I00 to I25° F., using four or fivechanges of one minute each, and they are then fixed for oneminute in:

Hypo .Sodium sulphite (dry) .Sodium bisulphite .Water to make .

240 g10 g25 g

1000 cc


Where the dichromatized relief images are stained after beingwashed out, the three elements in yellow, magenta and blue-green can be assembled to give a three-color transparency.Where the developed, bleached and washed-out images aremade, they are used for taking up acid dyes which in turn aretransferred to a piece of plain gelatin-coated paper which hasbeen treated with a mordant, as is practised in the well-knownWash-Off Relief process.

Chapter References

1. Charrion and Valette, Bull. Photo 6, 193, 1-10.

2. E. J. Wall, Photographic Emulsions, p. 188.3. T. H. Dixon and Co., Ltd., Letchworth, England.4. Product of the Atlas Powder Co., U. S. A.

CHAPTER XIPRINTING-OUT EMULSIONS

Salted Paper - Sensitizing Silk - Printing-out Emulsions - Gelatine-chloride Papers - Collodio-chloride Papers - Self-toning Papers - SilverPhosphate Papers

CERTAIN salts of silver will, if applied to paper, give a vis-ible image on exposure to light under a negative, which is

of sufficient density to provide a permanent print. Silver chlo-ride discolors or " prints out" a blue-violet color ; organic saltsof silver, such as the citrate, oxalate or tartrate, print out aredder color. The organic salts appear to be more sensitive tolight when in a humid condition, which explains why so-calledprint-out papers sensitized with a mixture of the two - as isusually done - yield a redder image if the humidity is high.None of these compounds will give a sufficiently intense imageby itself to withstand the fixing bath, and an excess of silvernitrate is required for building-up purposes. The visible imageconsists of metallic silver in a very fine state of division, and inthe heaviest densities there is probably not more than half amilligram of silver metal per square decimeter. The reducedsilver is colloidal in character, and some of it is held in solidsolution in the excess of silver halide; if, on prolonged exposure,the silver chloride becomes saturated, a deposit of bronze-colored silver metal may then appear on the surface in the deep-est shadows.

The simplest possible form of printing-out paper is the old-fashioned" salted paper," though this is not actually preparedwith an emulsion. In this process of sensitizing, the originalsurface of the paper is retained, and photographs printed on it

PRINTI G-OUT EMULSIO S 193

may be used as the basis for a drawing or for working up withwater colors, the silver image itself being removed afterwards.Any good quality drawing paper, such as Whatrnan's or Rive's,or goodunsized writing paper, can be sensitized. It must firstbe sized or coated with a solution containing the reacting salts:

Water 450 ccSodium chloride 15 gCitric acid 1.5 gArrowroot "...................... IS g

The arrowroot is mixed 'with about fifty cubic centimeters ofthe (cold) water, and rubbed into a smooth paste. The re-maining water with the sodium chloride and citric acid is nextadded, and the whole is brought to the boil, stirring continually,and boiled for three minutes. When cooling, a skin may formon the surface, and if so, it is removed and thrown away. Thesolution is then filtered. If a glossier surface is wanted, fivegrams of gelatin may be substituted for the arrowroot, when ofcourse boiling is not necessary, the gelatin being dissolved withheat and stirring in the usual way.

The salting solution as described can be applied with a wide,soft, camel's-hair or hog's-hair brush, or the paper may befloated upon it a sheet at a time and hung up to dry. It is a goodplan to make a mark in pencil on the back of each sheet indicat-ing that the opposite side has to be sensitized. If bubbles formwhen brushing on to the surface of a rough paper, a little alcohol(three or four per cent) may be used in the solution. The papermust be dried in a room or cupboard absolutely free from dust.

It is best sensitized the night before use by floating it, sheetby sheet, upon the silver bath, which is prepared as below:

A. Silver nitrate. . . . . .. . . .. . . .. .. . . . . . .. 20 gWater 100 cc

B. Citric acid IS gWater reo cc


A and B are mixed, and twenty cubic centimeters of pure grainalcohol added. The solution is used at room temperature, ina perfectly clean dish. A sheet of paper held by the edges isbent into an inverted U, the convex side brought gently intocontact with the surface of the sensitizing bath, adl::l the twosides then lowered until the sheet floats upon the liquid. Aftera minute it is lifted by one corner to see if there are any airbubbles, and if so, these must be removed and the sheet re-:lowered on to the solution for a further two minutes. Thickpaper may require four to five minutes in all. Rubber glovesshould be worn, or the fingers may get badly stained. Thesensitized paper is hung up to dry by the corners, in a warmroom or dust-free cupboard. All these proceedings can be car-ried out in Mazda light.

Printing is done in sunlight, and progress may be examinedby opening the frame a few feet away from the window. 'Whenall details are clearly visible, and the print is appreciably darkerthan is desired in the final image, the paper is removed from theframe and wa hed, toned in a gold bath, and fixed. If it is re-quired only as a guide for drawing, it may be simply fixed intwenty per cent hypo, but it will then take on an unpleasantorange color. The photographic image after drawing upon itcan be removed with Farmer's reducer.

If separate toning is to be given, it is imperative to wash theprint beforehand until all free silver nitrate has been removed'this will be apparent when the wash-water no lonzer becomesmilky through the formation of silver chloride. A .good toningbath is as follows:

Gold chloride. . . . . . . . . . . . . . . . . . . . . . . . . . . . . o.r gSodium aceta te . . . . . . . . . . . . . . . . . . . 2.8 gWater 400 cc

When the print has acquired a purple tone, it is rinsed and

PR! TIN G-OUT EMULSION S .195

fixed for five minutes in a ten per cent hypo solution, and isfinally well washed. By the use of a combined toning and fix-ing bath the print can be put without washing into the solution,toned and fixed simultaneously, and then finally rinsed. Sucha bath may be made up thus:

Water 300 ccHypo roo gLead acetate 0.3 gGold chloride. . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.r5 gPowdered chalk 7.5 g

Shake up well, stand overnight, and decant the clear solutionfor use.

SENSITIZING SILKS AND FABRICS.- For printing photo-graphs upon silk or other fabric, sensitizing is best clone withthe substitution of Iceland moss for arrowroot, starch, or gel-atin. This is a dried seaweed collected on Atlantic coasts, andis made into an " infusion" by mixing 2.5 g of the moss with500 cc of boiling water. It is decanted and filtered, and whencold is of a syrupy consistency. To it is added:

Sodium chloride . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 20 gGlacial acetic acid 50 cc

The mixed solution is used in a dish, and the pieces of materialare gently lowered on to the surface, the object being to coatthe surface of the material only, and to prevent as far as pos-sible the solution from penetrating through to the back. Aftertwo minutes' flotation, the pieces of silk, etc., are lifted by twocorners and hung up to dry on a stretched cord, and left untilthey no longer smell of acetic acid. They are then sensitizedby floating on:

Silver nitrate 25 g

Citric acid .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 20 gGrain alcohol 20 ccWater to make 250 cc

B. Silver nitrate .Ci tric acid .Water ........ ,- , .

750 cc2·5 g5 g

80 g

2$ gla g

250 cc

/.PRI TING-O T EMULSIONS 197

Solution B is added to A in a slow stream while the crock iskept stirred, both solutions being at I05° F. (400 C.). Themixed emulsion is almost transparent at first, but soon becomesturbid. After about ten minutes, forty to fifty cubic centimetersof denatured spirit is added, and slowly, with stirring, twentycubic centimeters of a 2.5 per cent solution of chrome alum.The emulsion is filtered through felt, or two thicknesses of linen,and coated at once. It cannot be set off and remelted. In-crease in the amount of citric acid tends to give higher contrast;substitution of an equivalent amount of ammonium citrate forthe Rochelle salts (sodium potassium tartrate) tends to givefuller half-tones. The addition of ten cubic centimeters of atwenty-five per cent solution of stronger ammonia water willgive increased sensitivity and somewhat bluer tones. Whenammonia is used, however, the emulsion should be stood afterits addition for half an hour at I05° to IIOo F. before coating.To mat the emulsion, fifty grams of rice starch, ground to apaste with a little water, is added before filtering. The matemulsion is best coated on glossy baryta paper.

Another reliable formula for a printing-out emulsion is thefollowing:

A. Gelatin 24 gAmmonium chloride 0.8 gWater : 200 cc

B. Tartaric acid - .Sodium bicarbonate : .Potash alum .

. Water .

0.8 g0-4 g0·5 g

30 cc

C. Silver nitrate , . . . . . . . . . . . . .. 10 gCitric acid gWater 45 cc

A and B are heated to r ro? F. and mixed; solution C is thenadded slowly with stirring, at the same temperature. Final ad-


A high-contrast negative should be used for printing ('Y 2 to2·5), and the image printed deeply and then treated with a com-bined toning and fixing bath. Wash and dry and smooth outcautiously with a moderately hot iron.

Plain salted paper as above described was follow~ by a muchmore stable product, prepared with a coatinz of salted albumenand sensitized afterwards with silver nitrate, which held thefield for many years. After this era of albumenized paper camethe far more practical gelatino-chloride paper popularly knownas printing-out paper in America, and as P.O.P. in England.This was baryta paper coated with a single emulsion of silverchloride in gelatin, containing a large excess of silver nitrate,and it had both before and after use comparatively highkeeping qualities. It printed rapidly, gave a superb range of~one, a~d could ~e toned with gold, platinum or palladium, giv-lI1g an Image which would last for ten years or more.

The need for an excess of silver nitrate makes it necessaryto select paper of great purity for coating gelatino-chloride emul-sions, and fairly heavily baryta-coated rag paper has been used.After printing-out in daylight, the excess of free nitrate is re-moved by washing, and the prints are then toned and fixed. Toavoid the trouble of toning, a soluble gold salt such as the thio-cyanate was incorporated in the emulsion itself makinz self-, otoning papers which acquired a gold purple tone during fixation.

One form of print-out emulsion giving brilliant prints butof only moderate keeping quality is made as follows:

A. Water .Ammonium chloride .Sodium potassium tartrate .Gelatin ................... · .... " ... ,'


ditions are twenty cubic centirneters of grain alcohol and fivecubic centimeters of 2.5 per cent chrome alum solution.

It has been suggested that the decomposition of the gelatinby free silver nitrate can be obviated by the use of,."fLsufficientamount of organic salt, such as ammonium or potassium citrate,to take up the excess of silver nitrate over the combining weightof chloride. Thus for roo grams of Ag 03, r8.3 grams ofsodium chloride and 29 grams of potassium citrate would beused, keeping the citric acid at about sixty per cent of the silver.

The use of collodion in place of gelatin offers some attraction,and collodio-chloride printing-out papers have enjoyed consid-erable popularity. The drawback to the use of collodion inlaboratory experimentation is its very high inflammability. The,solvents used are not only inflammable to a marked degree, butwhen diluted with air may form highly explosive mixtures. Infactory practice not only can fire risks be eliminated by suitableprecautions, but methods are adopted of recovering the solventsfor use again.

A formula which may be made up successfully in small quan-tity is given below:

A. Pyroxylin or " celloidin" 10 gEther (methylated) 200 ccAlcohol 200 ccCastor oil cc

B. Lithium chloride .Strontium chloride .Alcohol .

C. Silver nitrate .Distilled water .Alcohol .

I.2 g1.2 g

35 cc

10 CT<>

10 cc25 cc

2·5 g35 cc

D. Citric acid .Alcohol " .

Mix C and DJ and add gradually to the collodion A in a large

PRI TI G-OUT EMULSIO S 199

flask, with constant agitation. Then add B very gradually, withconstant shaking. Lastly add 2.5 cc glycerin. Stand fortwenty-four hours before coating on baryta paper.

An alternative formula 1 runs as follows:

A. 4 per cent collodion 500 cc

B. Alcohol 50 cc10 per cent aqueous solution of strontium

chloride 3 ccla per cent aqueous solution of lithium

chloride I cc

C. 50 per cent solution of silver nitrate. . . . . .. 24 ccAlcohol 50 cc

D. 25 per cent aqueous solution of citric acid.. 60 ccAlcohol 25 cc

A and B are thoroughly mixed at room' temperature, a two-literPyrex flask being very convenient. C and D are then mixedtogether and added to the mixture in the flask, ten to fifteencubic centimeters at a time, with vigorous shaking. When emul-sification is completed, the following finals are added:

50 per cent castor oil in alcohol. . . . . . . . . . . . . .. 3 ccAlcohol 25 cc50 per' cent glycerin in alcohol. . . . . . . . . . . . . . . .. 8 cc

This emulsion can be made self-toning by adding 0.15 grams ofgold chloride dissolved in ten cubic centimeters of alcohol; theaddition of ammonium thiocyanate or lead acetate or nitratehas also been suggested. Self-toning emulsions are u ually un-stable and should be coated at once.

The coating of collodion emulsions for experimental purposesis by no means easy. Baryta paper can be unrolled on theemulsion placed in a dish, as has been described elsewhere(p. 163), but the film may form in tiny wrinkles and give abroken surface. Small pieces can be coated individually by

PRI TI TG-OUT EMULSIONS

cubic centimeters of phosphoric acid of specific gravity 1.265was added to 200 cubic centimeters of collodion, together with7 grams of citric acid previously dissolved in 14 cubic centi-meters of alcohol. This was then mixed with:

Silver nitrate 10 gWater 125 ccAlcohol 20 ccWhen dissolved, add Lithium carbonate. .. . . . 7·5 g

When the evolution of carbonic acid has ceased, 1.5 cubic centi-meters each of glycerin and grain alcohol are added.

Silver phosphate images may be developed with a weak solu-tion of metol acidified with acetic or tartaric acid.

Chapter Rejerences

1. A. Truum, Die Herstelluuz photographischer Papiere, p. 300.

2. E. Valenta, Phot, Korr., 37, pp. 313, 419, 1900.


turning up the edges about a quarter-inch all around, laying thesheet on glass, and coating as a glass plate. If an excess ispoured on, and after setting Ior a few moments is poured DD, thepaper may be held at one corner and given a zigzagsmovement toprevent wrinkling. The warning against the high inflamma-bility of collodion and its solvents, especially when used in opendishes or during evaporating when drying the paper, must againbe emphasized. Positively no naked lights may be used, nosmoking, and no use of electric heaters having bare resistanceelements. The room should be kept cool, and should be wellventilated.

Stripping-collodion may be made by first coating baryta pa-per with an eight per cent gelatin solution, then with the col-lodion emulsion. The gelatin coat is, of course, first dried.After the print has been toned and fixed, immersion in warmwater will soften the gelatin, and the collodion image can befloated off.

While printing-out papers have gone out of fashion on ac-count of the convenience of development papers and theirgreater workinz rapidity, they are capable of giving exceedinglybeautiful results, and there is some indication of their re-introduction. The experimenter. in emulsion work will certainlyfind a great deal of fascination in the printing-out field, andample scope for investigation.

SILVER PHOSPHATE SENSITIZERS.- Phosphate of silver isformed by the reaction of sodium phosphate, Na2HPO'1l or tri-basic sodium phosphate, I a3PO'i' with silver nitrate. It is moresensitive than the chloride, and must therefore be dealt with inyellow light. It gives a somewhat exaggerated latitude, so that'it is best suited to hard, vigorous negatives. A precipitate ofsilver phosphate dissolved in an organic acid, such as citric acid)can be applied with a brush to raw paper, or the salt itself maybe precipitated in collodion as suggested by Valenta." Here 2.9

20I

CHAPTER XIITESTING EMULSIONED PRODUCTS

Testing Equipment - Estimation of Speed and Quality - Interpretationof the Characteristic Curve - Measurement of Color-sensitivity - Pho-tometers and Density Meters - Keeping Tests

A N important part of the routine of sensitive material manu-fl.facture lies in the adequate testing of the finished products.In any form of investigation, exhaustive tests of the coatedplates, films or papers will be more productive of results thanthe making of numbers of more or less random experiments.Some form of sensitometric test is required, and the characterof different emulsions must be deduced and compared. Sen-sitometric work demands suitable apparatus, which may besomewhat costly, but it must be regarded as absolutely indis-pensable and a very sound investment where commercial workis envisaged. Where the products are being made for experi-mental purposes, and do not have to compete with other manu-factured articles, a great deal can be accomplished with the aidof more modest equipment.

The main characteristics of plates, films, and papers are mostsimply represented by the type of curve originated by Hurterand Driffield, which shows the relationship between the photo-graphic response of any individual material after developmentand the exposure to light required to produce it. As we haveseen, emulsions are, in general, sensitive not only to visible lightof 400 to 700 mu, but to radiations of shorter waveIength, espe-cially the ultra-violet, X-rays and the radiations emitted byradioactive bodies. A developable effect is also produced bya static discharge in contact with the film, and to some extent

TESTING EMULSIONED PRODUCTS 203

by pressure. Bloch states 1 that pressure or abrasion marksrender the emulsion insensitive to subsequent light action.

The developed image consists of reduced metallic silver grains,and the mass of reduced silver per unit area is proportionalto the logarithm of the opacity. Opacity is the ratio of the in-cident to the tr~nsmitted light, or the reciprocal of transmission,

~. The need for expressing it logarithmically can be seen fromTthe simple table showing that the quantity of light transmittedby an increasing number of layers of an absorbing material eachone of which absorbs half the incident light, proceeds geometri-cally and not arithmetically, as under:

Proportion of lighttransmitted Opacity

1 layer2 layers3 layers4 layers5 layers

11~i=i1~t=ii~i=hI~h=*

48

16

32

and so on. Expressed as logarithms to base IO, we have (ap-proximately) ~

log (opacity)log 4log 8log 16

log 32

(2 = 100.S)

(4 = 10°·6)

(8 =;= 10°·9)(16 = 101.2)

(32 = 101.5)

2 =0.3=0.6=0·9= 1.2

= 1.5

and so on.The logarithm to base IO of the opacity is termed the density.

The density of the coated film bears a constant relation to thelogarithm of the exposure over a range of illumination I X Twhich is dependent on the particular emulsion. The curve con-necting density with log opacity, usually referred to as the H

204 PHOTOGRAPHIC EMULSION TECHN IQUE

and D curve after Hurter and Driffield, or the characteristiccurve, takes the general form shown in Fig. 59. It will be seen

3 <,/ -Q

//

/INERTIA V,R T I T U D E:

---:V I

,

2

Ao 3 B1 2LOG e:

FIG. 59

that the increase in density for increase in log exposure is notequal for the lower exposures, but is progressively greater asexposure rises, and that the shape of the curve is curved in theselower densities, known as the foot or toe. After exposure hasexceeded a certain amount, however, density increases by regu-lar amounts over a period of correct exposure represented bythe straight-line portion PQ, this part of the curve giving us ameasure of the latitude of the emulsion. On very considerableexposure, the curve is seen to turn over, when still further in-creases in exposure (ovel'exposure) actually produce less dens-ity in tead of greater. This is the region of solarization orreversal, and is usually encountered at an earlier stage in the

TESTIN G EMULSIONED PRODUCTS 205case of fast large-grained emulsions than it is in slower, morefi ne-grained emulsions.

The characteristic curve gives us an immense amount of use-ful information, enabling us to make a very complete post-mortem of an experimental emulsion, or the effects on its qualityof any variations which may have been made in the course ofexperiment. It will be seen in Fig. 59 that on extending thestraight-line portion to cut the log exposure axis, it cuts it at apoint, A. The length OA represents the inertia of the material,and this is inversely proportional to the so-called speed. Itmust be emphasized.ihowever, that the characteristics exhibitedby any such Hand D curve, and the inertia and speed deducedtherefrom depend upon the particular developer used, on thetime of development and its temperature, and - to some extent- on the spectral quality of the light used in exposure.

If log exposure is spaced along the horizontal axis equallywith density along the vertical, so that for example log exposureof 3 measures the same as density 3 on the graph, then we ob-tain a universally recognized means of measuring contrast orgamma, 'Y. If the straight-line portion of the curve be at anangle of 450 with the log E axis, then gamma = tan 4S0, or I.

Generally, 'Y= tan 8, where 8 is the angle made by thestraight-line part of the curve with the log E axis.

As contrast depends' on the amount of development, and in-asrnuch as the rate of development depends on the temperatureof the solution, it is usual practice to publish Time/Tempera-ture/Gamma curves. An exhaustive series of these will befound in Morgan and Lester's " Photo Lab. Index," measuredfor practically all marketed materials. Such curves are alsobeing extensively published by manufacturers, and their prep-aration and issue with new sensitized materials should be re-garded by new entrants into the manufacturing field as in-


dispensable. An example of the form in which sensitometricdata is supplied is seen in Fig. 60, reproduced from "KodakFilms."

10 MINS. 6

3·0 2'0 --+----::7"'--t-----r--r------;r----t-

MIt-.lICi.

,.Ol------r----/-+--l-/----/-'---t----_--t-

1.0 LOG E o '·0

FIG. 60

In Fig. 6r we see a number of curves plotted for one material,developed for increasing times. The contrast is low at shortdevelopment, rising with increasing time of development untilthe top curve is reached after a aradual falling off of the rise withtime. In this case after eleven minutes at 70° F., longer de-velopment fails to increase contrast. In other words, gammainfinity has been reached, denoted by ,,/00. While, therefore,gamma is a measure depending on a particular developing solu-

TESTING EMULSIO ED PRODUCTS 207lion, its temperature, and the time of development, gam~a isa concrete factor which gives a definite idea of the ultimateperformance of an emulsion. Gamma infinity is largel~ in-dependent of the wavelength of the light used in exposure If the

2

2op LOG. £

FIG. 6r

plate be developed to gamma infinity 2, whereas a calor-sensitiveemulsion not developed to ,,/00 is definitely dependent on wave-le~gth.* It i for this reason that the yellow printer negativein one-shot camera work (exposed through the blue-violet fil-ter) is generally given a time of development increased fromtwenty-five to one hundred per cent over the times allotted to

* Certain recently introduced emulsions of the panchromatic type areclaimed to give a con istent gamma throughout the spectrum, and the yel-low, red, and cyan printers can thus be all given the same time of develop-ment.


the red and cyan printers. In cases where narrow-cut filtersare used in separation work from color transparencies, thetime of development should actually be varied for each ofthe three. Gamma infinity being an important fa~tor from anemulsion-making point of view, the following simple formulafor deducing it from the gammas obtained for two times of de-velopment, 1'1 for a time T, and l' 2 for a time 2T, one doublethe other, is given:

'Yoo = (2'YI - 'Y2)

In using this formula, the higher of the two gammas should notbe too close to l'00 •

It will be noted that the family of curves shown in Fig. 60all meet at a point below the log E axis if extended. This isaccounted for, according to Nietz," by the presence in the emul-sion of free bromide, without which the meeting point wouldbe substantially on the log E axis. Practically all commercialbromide emulsions contain free bromide, which not only min-imizes general fog, but assists in preventing edge formation offog on coated plates.

It has been stated 1 that" the method devised by Hurter andDriffield in 1890 for studying the properties of a photographicemulsion has been extremely fruitful and still furnishes the mostcomplete information on the different properties of sensitiveemulsions, and represents them graphically under the most fa-vorable conditions. At the same time the mode of numericalexpression of sensitivity proposed by them, (H and D speed)has now scarcely any significance. At the time of Hurter andDriffield, already long past, the different types of emulsions allhad characteristic curves of the same general form, and in orderto express the relative sensitivity of two plates it was sufficientto measure (parallel to the exposure axis) the distance between


the corresponding curves after the two plates had been developedto the same gamma. Since 19 IO, however, new types of emul-sions have appeared with 'characteristics very different fromthese old emulsions, and it is impossible to imagine a singlenumerical value which defines at the same time the form of acurve and its position relative to another one."

A simple example of the position can be seen in Fig. 62. This

O~~~--~~--------~--------~3LOG. E

FIG. 62

is a curve' representative of many super-speed emulsions. Itmay be considered as having two straight-line portions, theearlier of them really a prolonged foot. If we extend the footportion, AP, to meet- the log E axis, it gives an inertia of Op;whereas if we extend the straight-line portion, PQ, it meets thelog E axis at q, giving an inertia of Oq. The speed of theemulsion, obtained by dividing Op into a constant as would bedone in Hand D practice, is obviously far greater than the

2IO PHOTOGRAPHIC EMULSION TECHNIQUE TESTING EMULSIO ED PRODUCTS 2I1

speed measured by dividing Oq into the same coristant. Yet thestudio portraitist may do all his work with low densities onthe foot portion and find the material extremely fast, while theman who wants heavier densities must use the P(1.part of thecurve, and will find the emulsion quite moderate in speed. Thiscase is typical of mixed emulsions, and can occur to a very ap-preciable extent in the case of split silvers, where one silver isre-dissolved and the other added plain, etc. The emulsion-maker's art is to provide a smooth curve with long straight-line portion, no matter how he divides his silvers and manipu-lates his ripening.

The broad Hand D method is nevertheless generally usedin the United States for sensitometric work. A typical exampleof its application is seen in the motion-picture industry wherea staff is continuously engaged in making sensitometric testsof all film stock used for camera work, sound recording andprinting, and for controlling and checking developing and othersolutions for performance, exhaustion and so on. In manybranches of technical photography, small differences in theshape of the toe of the characteri tic curve may have the highestignificance, as for example in sound recording, radio fac-

simile and photo-engraving. Jeedless to say, refined sensito-metric work of this type is not po sible without adequate labora-tory plant and physical apparatus, and more particularly in theabsence of machine-coated materials.

In experimental work with emulsions, a great deal of usefulinformation can be obtained as to the effects of variations intreatment, modifications in formula, and so on, with a goodstep-wedge, a stable light source, temperature-controlled de-velopment, and some dependable form of densitometer. Thestep-wedge provides us with an intensity scale of exposure, asagainst the time scale of the Hurter and Driffield or Scheinersector wheels. The usual sector wheel of Hurter and Driffield

has nine slots (Fig. 63), each one subtending double the angleof the previous one, thus giving an exposure range of I: 256, theratio being X 2 for each step. The disc \V (Fig. 64) is mounted

vertically on a shaft driven by motor. A plateholder P, behindthe upper half of the disc, receives light through the slots asthe disc is rotated- by a driving wheel, D. A controlled lightsource, L, filtered to daylight and thoroughly screened, is placedat a fixed distance - usually one meter - from the sectorwheel, which may be turned rapidly (when the intermittencyeffect creeps in), or through one complete turn for the entireexposure. The lamp; L, which must be housed in a dead-blacked box with suitable aperture facing the sector wheel ofsay one inch square, is run from a storage battery B, througha resistance R, with regulating contact K. An accurate volt-meter such as a Weston meter sensitive to within ± 0.2 per


cent, is shunted across the lamp terminals, and the resistanceis set to give, say, 5.8 volts with a c-volt lamp. This slightunder-running tends to give uniformity and to increase the use-ful life of the lamp. Hurter and Driffield used! a " standardcandle" as their light source; Eder subsequently adopted asmall benzene lamp; Sheppard and Mees employed an acetyleneflame burning under constant pressure. Today secondarystandards are most conveniently obtained with incandescentlamps, but it must be understood that photographic sensitive-ness varies quite considerably with the spectral quality of the

D

w

p

li KII~

FIG. 64

light used, so that this must be correctly defined and properlycontrolled. The spectral energy distribution of incandescentlamps varies greatly with filament temperature, which in turnvaries with applied voltage.

Probably the most useful type of step-wedge is one having

TESTING EMULSIONED PRODUCTS 213twelve steps - no density at the first (d = 0), complete opac-ity at the twelfth (d = 00 ), and ten intermediate steps of:

Density 0.3 baving approximately transmission 1120.6 1/4o~ 1/81.2 1/16

1.5 11321.8 1/642.1 r/1282-4 112562.7 1/512

3.0 1/1024

The Eastman Kodak Company supplies wedges havingtwenty-one steps with densities ranging from 0.15 to 3.0, thestep increments being o. I 5 instead of 0.3, the opacity ratio ofsuccessive steps being thus X Y2 instead of X 2. The IlforclCompany, Ilford, London, England, specializes in step-wedges,making them to any- reasonable specification. A four by fiveinch step-wedge with ten steps costs fifteen to twenty dollars.Alternatively, by obtaining some neutral gray gelatin of threeor four densities, a step-wedge can be built up on a piece ofglass, using different combinations for the successive steps.Eight strips one-half inch deep and four inches wide can bemounted ori a 4 by s-inch glass, leaving the top half inch clearto give total transmission and blocking out the last half inchwith a strip of dead black paper to give total opacity.

The wedge must be used in a solid plateholder or frame ofample dimensions, as shown in Fig. 65, which can be placedon the bench at a measured distance from a suitable and con-trollable light source, such as a 6 or 8-volt motor headlamprun from a storage battery with a potentiometer at a voltagepreferably of rather less than six volts. A tungsten-to-daylightfilter such as the Wratten 78 will filter the light and give re-sults more closely approximating practical daylight use.

214 PHOTOGRAPHIC EMULSIO T TECHNIQUE

Sensitometry connected with emulsion-making experimentsis not quite on a par with the general testing of photographicmaterials, as in most cases comparisons of speed and character-istics of members of a series of similar emulsions are the chief

FIG. 65

concern. Relative speeds can be obtained by means of thepoints on the log E axis at which the straight-line portions ofthe curves, when prolonged, intersect it. The antilog of thedifference between the log E readings will give the speed ratio.Contrast can be estimated by measuring the slope of the.straight-line portion of tests developed to gamma infinity, andthe materials can then be classified for general reference intogroups such as:

FlatNormalMediumContrastyHardExtremely hard

when "rce is less than I.2

when 'Y00 lies between I.2 and 1.6

when "rcc is greater than 2.0

when 'Y 00 is above 2·7

when 'Y00 lies between 3·5 and 4.0when "y co lies between 4.0 and 5.0

Such comparisons may be made with any stable light source,and the necessary exposure to give a reasonably straight-lineportion of the curve. If, on the other hand, densities be plotted


against exposures measured in lumens per square meter Xseconds, or measured from a light source of known candlepower of geometrically decreasing strength as obtained with awedge, comparative speed determinations can be made withconsiderable accuracy. The step-wedge which can be used ina plateholder at a fixed distance from a standard light source,which distance can be varied with slow or extreme speed emul-sions, is probably the best for experimental work, while anaccurately cut sector-wheel with a satisfactory secondarystandard light source is more generally u -eful in works labora-tories.

In view of the very high humidity experienced in many partsof the country, and the utterly inadequate means of dealingwith it which are found in' most amateur laboratories, attentionshould be drawn to the effect which humidity may have on sen-sitometric tests. The importance of this effect was emphasizedby Renwick," in connection with .the testing of negative andpositive motion-picture film stock. A simple method of partialcontrol in use by the author may be recommended. This isto place the samples of sensitive material to be tested in a largedesiccator for forty-eight hours before they are required, usinga saturated solution of sodium chloride (kitchen salt) in thebottom. This maintains an " atmosphere" of about seventy-five per cent relative humidity, which is more or less representa-tive of average conditions in summer time in the Jnited States.It was found by Dimitroff 6 that the background density inmany cases increases with decrease of humidity, and that insome cases the graininess of an emulsion appears lessened if theplate is treated with dry air.

The control of development is of prime importance. It hasbeen pointed out that the characteristic curve depends uponthe particular developing formula used, so that the need for astandard solution, used at a defmite temperature, is self-evident.


The old pyrogallol-soda formula recommended for a long timefor Hand D de terminations, should be replaced by such a for-mula as D- II, D-72 or one of the fine-grained type, althougha developer without bromide is desirable for investigation work.If a number of comparative sensitometric tests be made, it isimportant that they all be made together for the same time andwith as nearly as possible the same amount of agitation. Forsuch a purpose the use of a Dewar vertical vacuum flask maybe recommended, where three or four strips can be attached byrubber bands to the sides of a triangular or rectangular stickof wood and immersed in the temperature-controlled solution.

To recapitulate, the emulsion chemist should use, for eachfamily of tests,

(a) a constant developing solution, freshly made.(b) a constant time of development.(c) a constant temperature.

Where a gamma/time curve is required, conditions (a) and(c) must be enforced, the time alone varying, with a short-stopbath used between development and fixing or "the immediatetransference of the test from developer to a strong acid fixingbath. A vertical developing tank standing in an outer bath ofwater is recommended for gamma/time work, the tests beingsuspended vertically in dilute developing solution, giving ofcourse proportionally longer time. The liberation of halogen inthe process of reduction of the silver bromide during develop-ment and its conversion by the accelerator into alkali bromide,will restrain development in adjacent strips and give erroneousor streaky densities unless proper agitation is applied. De-veloping sensitometric tests in a flat dish and removing the by-products of development by gentle brushing with a soft camel's-hair brush, has been recommended. Agitation of the plate orfilm, however, increases the rate of development, so that meansmust be adopted of giving an average amount of agitation to

TESTING EMULSIO ED PRODUCTS 217

each batch of tests. Much valuable information on the effectsof agitation has been published in Eastman Kodak researchcommunications.

Densities can be measured directly by means of their ab-sorption of a beam of light as recorded by a photo-electric cell.Articles describing the construction of densitometers of this

FIG. 66. DI(\GRAM OF CAPSTAFl' DENSITOMETER OPTICAL SYSTEM

type have appeared many times in recent photographic litera-ture. Two' beams of light may be compared in another typeof instrument, one passing straight through the density to bemeasured, the other being extinguished by suitable means untilit matches it. Such instruments depend on the use of crossed

icol prisms, or on the use of a graduated gray wedge. TheHi.ifner spectro-photometer and the Hilger- utting spectro-photometer are examples of the first-named, while the densitymeter worked out by the British Photographic Research Asso-ciation depends upon matching the direct and the wedge-extinguished beams by means of a photo-cell.

Another type of densitometer throws the two beams from the


FlG.67. CAPSTAFFDE_'SITOMETER

density patch and the gray wedge into an eyepiece, the wedgebeing brought to a position where it equalizes them visually.The position of the sliding or revolving wedge is read off ona calibrated scale in densities direct. An instrument of thistype, due to Capstaff and Purdy," is manufactured by the East-man Kodak Company; a more elaborate model is made withwhich reflection densities as well as transmission densities can

TESTING EMULSIONED PRODUCTS 219be measured. The Capstaff instrument measures silver depositsfrom a density of 0 to 3.0, and will deal with areas of only one-half square millimeter, being particularly designed for use inthe motion picture industry. The optical system is seen inFig. 66.

Light from source A illuminates at H the density to be meas-ured, and furnishes the comparison beam. A photometric fieldat G is obtained by the aid of mirrors B, D, F, and G. W isa circular wedge, shown in Fig. 68, for decreasing by a knownamount the intensity of the light illuminating the density. ]is an eyepiece for viewing the field, in which the density is seenas a central spot in a circle of variable density; when the twobeams are matched, the central spot vanishes. The combined

FIG. 68. DENSITOMETER HEAD RAISED SnowING CmCULAR WEDGE


densitometer for measuring negative or print reflection densitiesis seen in Fig. 67.

In connection with density measurements, it must be bornein mind that the reduced silver grains scatter th~ light to anextent depending on the emulsion characteri tics, and that adensity reading obtained by a beam of parallel rays will differfrom that obtained by diffuselight. Diffuse density meters havea small opal or finely ground glass directly under the plate orfilm so that the light is completely scattered. The ratio ofdensity (parallel) to density (diffuse) can yield valuable in-formation as to the physical quality of the developed emulsion.

A simple and inexpensive wedge densitometer designed bySanger Shepherd and improved by W. B. Ferguson 8 is shownin Fig. 69· Light falls directly on the front of the inclined in-

FIG. 69

strument, as from a north window. One beam of light passesthrough the calibrated gray wedge, W, and is reflected into theeyepiece, E, by the mirror A. The other beam passes throughthe density patch to be read (laid over the middle aperture),and is reflected by the mirror, B, so as to appear in the eyepiece


contiguous to the· first beam. Both beams are scattered byopal glass. The wedge, W, is slid along until the two beamsappear of equal brightness. Readings can be obtained with anaccuracy of about ± two per cent.

A simple photo-electric densitometer devised by Schoen isseen in Fig. 70. E is a lamp run at controlled voltage by means

FIG. 70

of a potentiometer. A is a slot oyer which the density patchis placed. C is a thalofide cell (Case) coupled up with a galva-nometer and battery, B. The galvanometer, G, is brought tothe null point with the shunt circuit shown, and the densitythen introduced at A and measured by means of a previouslycalibrated dial or by a table prepared from known densitymeasurements, or by interpolation on a prepared curve.

If an emulsion be exposed through a small circular hole placedin contact with the film, for increasing ·times of exposure, thediameter will spread as exposure is increased, and the slope ofa curve connecting diameter with exposure was termed by Gold-berg the turbidity factor; this was found to be zero in the caseof Lippmann plates (p. 135), although no absolute relation be-tween the factor and the grain of a plate was established. Thesharpness factor of an emulsion has been given by Goldberg as

gammaturbidity factor

222 PHOTOGRAPHIC EMULSION TECH IQUE'·5r-------,-------,-------,--- .- ~

'.O!------i-------t~L!~~----~~----~

o LOG E1 "5 2 2'5

FrG. 71

Translated into practical language, an emulsion to give sharpimages should have a high development factor (gamma) andlow turbidity, a long straight-line portion to its characteristiccurve, and should rise to the maximum slope at as Iowa densityas possible, i.e., not have a long, rounded toe.

So far, we have been dealing with measurements of trans-parent images only, such as negatives or transparencies, on glassor celluloid. When, however, we come to deal with developmentpapers, we must make measurements of the light reflected fromthe surjace of the image. The contrast as seen by reflectionincreases rapidly at first, and then becomes stationary, all thedeeper tones being lost or merged in papers having too high acontrast when fully developed out. This is seen from the char-acteristic curves given by Mees (Fig. 7I), for a bromide paper.It will be noticed that the latitude and gamma remain fairlyconstant over most of the range, due largely to a considerableregression of inertia with increasing time of development, in-dicating that good prints can be obtained of varying contrast

TESTIN G EMULSIO ED PRODUCTS 223

by a well-coordinated variation of development time and ex-posure.

For sensitometric tests of development papers, a reflectiondensitometer is needed, and probably no more convenient oneexists than the Eastman Kodak meter already referred to, ofCapstaff and Purdy. An excellent densitometer is made, how-ever, by W. Watson and Sons, Ltd., of High Barnet, Herts, Eng-land. This is the outcome of many years' work with the originalselenium instrument designed for the British Photographic Re-search Association by Toy and Rawling." The design of thenegative density meter is seen in Fig. 72. C is a photo-electriccell and W is the neutral gray wedge. A carrier, E, holds thecell' and is provided with suitable adjustments for bringing twowindows into correct alignment in relation to two diffusing opals01 and O2, The wedge is held in a U-shaped bracket which is

FrG. 72


fixed to a slide provided with a rack, 111,by means of which thewedge can be moved across the light beam. The other end ofthe bracket carries a vernier which moves on the scale S. Asingle light source, F, is used. The light beam is &lternately di-rected by means of a lever on to the cell through the density

D

••20 ~

~1.0 t5

0.2 0.6 1.0 1.4 1.8 2.2

FIG. 73 FIe. 74. NEGATIVE RANGES OF

CONTACT PRINTS

patch and through the wedge, until a spot of light from a gal-vanometer in circuit with the cell remains at zero, indicatingthat the negative density equals the wedge density. The instru-ment has been elaborated to read reflection den ities and to re-cord directly on graph paper, and can be strongly recommendedfor works practice. The cost of the density meter, galvanom-eter, lamp and scale, single-tube amplifier and automatic plot-ting device is, in England, $625.

Some care must be taken in considering reflection densitymeasurements because the character of the image can be con-siderably affected by the surface of the paper, the coating ofthe emulsion, and the type of pre-coating (baryta coating orsizing) used before the application of the emulsion. The effectof the non-stress coating should be negligible. The coating

TESTING EMULSIO_ ED PRODUCTS 225

weight can also affect the differentiation of the heavy densities,which are easily la t by too much silver. In dealing with anyserious sensitornetric work on development papers, the readeris particularly referred to the papers of L. A. Jones on paper con-trast 10 and to the Hurter and Driffield Memorial lecture (I926),of F. F. Renwick.P The latter suggests a determination of theapparent integral contrast of a paper calculated from tangentsread at not fewer than ten equal intervals along the character-istic curve, covering the whole useful range of the paper. Figure73 shows ten such points, the black spot in the middle indicatingthe averaze at which the contrast factor would be taken. Ao

chart connecting negative ranges and contrasts of seven dif-ferent grades of paper measured in this way is seen expressedin Fig. 74.

Development papers do not cover the entire range of tonesin the subject. While Jones finds that chloride papers foramateur photo-finishing have a short scale of between I: 5 andI: 20, Renwick finds a I: 20 range for mat surface papers andas much as I: 50 for glossy. Reproduction of the tone valuesof a negative must- necessarily be limited by the straight-lineportion of the characteristic curve of the printing paper, and asthe range of reflectance in this region is considerably less thanthe overallrange, some sacrifice must be accepted at both endsof the scale.

The great attention now being devoted to the making of calorprints on paper naturally introduces the problem of measuringcolored images by reflected light. A step-wedge is used in themanner already described, but colored steps on Carbro, Ch rom-atone or Wash-Off Relief paper are measured as greys by in-terposing in the light beam a filter of calor complementary tothat of the strip concerned. Thus in measuring a magenta strip,a zreen filter (minus magenta) would be interposed in the light

o d .beam or in the eyepiece. Such readings enable us to eterrnine


the effect on gamma of variations in the pH of dye baths usedin Wash-Off Relief; the effect of modifications in color ton-ing, imbition, etc., on density range, latitude and contrast; andthe effects of concentration of dichromate in segsitizing car-bon tissue; and so on. In the case of silver halide emulsions,tests coated on glass are of great additional value, as theyare not obscured by the physical characteristics of the papersupport.

COLOR-ENSITIVENESS. - An important field of investigationin the testing of photographic materials is the analysis or re-cording of their response to color. This must be done by ex-posing the emulsion to the various colors of the spectrum andmeasuring its response by the density recorded in different re-gions. It is somewhat unfortunate that anything in the wayof a spectrograph or means of photographically recording colorresponse should be so prohibitive in cost, when an excellent andmoderately accurate instrument can be made with a little skillfor a few dollars. The simplest form of spectrograph is prob-ably that designed by A. K. Tallent, a diagram of which is seenin Fig. 75. Light from a lamp, L, is focused by a condensing

H

FIG. 75

lens C, on to a slit S, passing through a filter at F when de-sired, and through a small optical wedge, W, if wedge spectraare to be recorded. G is a prism-grating giving" direct-vision"dispersion so that an image of the spectrum falls upon a plate


or film in the holder, H. In front of the plate when exposed awavelength scale, Q, can be thrown in.

The author designed many years ago a simple spectrographcapable of doing thoroughly good work, many of which havesince been made and used in different factories for routinetesting. For the information of those who wish to build suchan instrument for themselves, the following particulars aregiven. The general design is shown in Fig. 76. A box of the

FIG. 76

approximate shape indicated is built up of seasoned mahoganyabout three-eighths of an inch thick. On a front about two bytwo inches, or a little larger, is mounted centrally the slit. Awooden block is made to take a short focus lens, L1• This maybe a cheap achromat of about five and one-half or six inchesfocal length. Immediately behind it is fixed a diffraction grat-ing replica. Celluloid casts of diffractions gratings, mounted onplane glass, are obtainable from Central Scientific Co. for aslittle as $2.50. The size is 25 by 21 millimeters, and thereare 5,900 lines per centimeter. The collimated light, paral-lelized by the slit being placed at exactly its own focal lengthfrom the lens, falls on the grating, the plane of which is parallel


to that of the collimating lens. The light, after passing throughthe grating, is diffracted, and will cast a first-order spectrum atan angle of about 18° or 20° on a screen, AB (in this case theemulsion); this must however be focused, and £06 the purposean objective lens, achromatic of course, is fitted behind thegrating in the manner shown, also at its own focal length fromthe plate plane AB. The longer the focus of this lens, the longerwill be the spectrum, and a lens of sixteen to twenty-two inchesfocal length is suggested for a four by five inch plateholder.The image of the slit will be magnified by the ratio of the focallength of the objective lens to the collimating lens, in this casesay 22/6, or 3.66, times. The dispersion, and therefore thelength of the spectrum, will depend also on the number of lines,per centimeter of the grating, increasing with ruling fineness.

FIG. 77

A four by five inch plateholder is a very convenient size, anda back must be fitted to the camera body such that it can beslid along for successive exposures. A manufactured slit isan expensive item, though always worth-while; a homemade slitwhich will work remarkably well can be made in the mannerindicated in Fig. 77. Two safety-razor blades, carefully paral-leled, are screwed down over a central hole about three-quarters


of a centimeter in diameter, with flat-headed screws. For testwork, the blades should be about a quarter of a milIimeter apart.The camera back must be solid except for a slot slightly smallerthan the spectrum, such as five-eighths or three-quarters of aninch wide and four inches long. Everything must be thoroughlydead-blacked, and to prevent scattered light from being reflectedon the plate during exposure, it is a good plan to fit a piece ofdead-blacked corrugated card along the bottom of the box.

Some sort of wavelength scale will be needed. This can bemade as follows. A light source such as a carbon arc is focusedby a condensing lens upon the slit, the poles being brushed witha solution of common salt, lithium chloride, a little calciumchloride, if possible a trace of thallous chloride, and a fewdrops of hydrochloric acid. A spectrum taken on a panchro-matic plate will give the following lines:

CalciumCalciumLithiumThalliumSodiumLithiumLithium

Violet wavelength 394 mu396 "460 "535 "589 "6ro "671 "

"BlueGreenYellmvOrangeRed

Measuring the distances between the lines for the various wave-lengths, a magnified scale can be drawn in India ink on Bristolboard, and filled in for wavelengths from say 380 to 7So mu.A great advantage of the grating over the prism for the dispers-ing medium is that the distance between lines is directly propor-tionalto the difference between wavelength, as against the ab-normal dispersion of the prism. As the plateholder is fiat andnot therefore equidistant along its entire length from the centerof the objective, a slight loss of definition is inevitable. Thescale is riext photographed down, on a process plate, so that thespectrum lines fall in their right places. This is ascertained by

230 PHOTOGRAPHIC EMULSIO TECHN IQUE

moving the negative over the focusing screen until the linescoincide. The scale when developed is cut out the exact sizeto fit easily into the slot in the back of the spectrograph, andmounted on a slider so that, when the plateholder ~ide is drawn,it can be pushed in as near to the sensitive film as possible.

It will be noted that many of the spectrograms published inthis book and in current literature are of the wedge form. Thisis very useful, as, if a logarithmic wedge be employed, it givesa measure of the relative sensitivity at the different wavelengths.A small gray glass optical wedge can be mounted on the slit,or a neutral gray wedge can be obtained to fit over the wave-length scale or slot. If the slot is three-quarters of an inch byfour inches, the wedge should be ordered the same size or a triflelonger, and graduating across the width from 0 to 2.5 or 0 to 3density. In the latter case the transmission is I to 1/IOOO.

A simple, but useful bit of calor-testing apparatus can bemade with a printing frame and a rectangular step-wedge,across the steps of which are gummed a number of strips ofgelatin filters, mounted side by side along the length of thewedge. Half a dozen such strips one-half inch wide and fourand one-quarter inches long would be used in the case of a wedgethree and one-quarter by four and one-quarter inches. A sug-gestion for the filters is:

W fatten No.Violet * . . . . . . . . . . . . . . . . . . . . .. 36Blue 50Blue-green 75Green 74Orange 58 + o. ISRed 29

* All violet filters pass a considerable amount of deep red. This can befiltered out by the use of a liquid cell of weak copper sulphate solution, theconcentration being adjusted by trial with the spectroscope so as to ex-tinguish the red up to about 650 m«.


This will give a useful comparison of the relative color sensitiv-ity of various materials, and of the relative sensitivity to indi-vidual colors of any particular material. Any such comparisonsare of course affected by the efficiency or transmission factorof the various filters.

COATINGWEIGHT.- It is important to be able to make achemical analysis of the amount of silver or silver halide in agiven area of plate or film. The following particulars are dueto Eggert and Noddack. One square decimeter (ten centimeterssquare) of the coated film or plate is taken. It is put into aporcelain dish and fifty cubic centimeters of a hot ten per centnitric acid solution is poured on it. To make this solution, tencubic centimeters of concentrated nitric acid and ninety cubiccentimeters of distilled water are mixed. When the emulsionis dissolved in the hot dilute nitric acid, twenty cubic centi-meters of a forty per cent solution of caustic potash is run in.This neutralizes the nitric acid. It is of extreme importancethat the nitric acid be completely neutralized, as the next step

is to run in thirty cubic centimeters of twice-decinormal (¥)potassium cyanide solution from a burette, stirring well. Anyexcess of nitric acid would liberate hydrocyanic acid at thisstage, the breathing of which is usually fatal. It is for thisreason that the excess of caustic alkali is first of all run in.When making up the standard solution of KC -, the cyanideshould be weighed out in a room where the ventilation is good,and not in a stuffy darkroom, on account of the noxious fumesthat it gives off.

When the silver halide has gone into complete solution, thecontents of the dish are transferred to a flask and the film orplate is rinsed several times with hot distilled water and therinsings added to the contents of the flask. A further small

232 PHOTOGRAPHIC ElVIULSIO TECH IQUE

quantity of the potassium cyanide solution (2 CC) is then runin, and 3 cc of the following indicator added:

IS per cent solution of potassium iodide 50 ccAmmonia (s.g. 0.910) 50 cc

The whole is then titrated back with standard decinormal silvernitrate solution until the end point - a faint persistent opales-cence - is reached. A similar determination must be madewith a blank; the difference between the blank and the actualtitration gives the amount of silver (AgN03) equivalent in thesample.

When positive or reversal type emulsions coated at beloweighty milligrams of silver halide per square decimeter are be-ing estimated, it is advisable to use two square decimetersrather than one of the material. It should be remembered thatpotassium cyanide is not anhydrous, and should always bestandardized against the silver nitrate solution. The reaction is:

AgBr + 2KCN= KAg(CN)2 + KBr

The potassium cyanide should therefore be of 2 molar concen-tration, or thirteen grams per liter. But in view of the salt notbeing completely anhydrous, fifteen grams per liter may beused. If IS.74 grams of silver nitrate per liter be used in mak-ing the decinormal standard silver solution instead of I 7.0 grams,the titration will read directly in milligrams of metal.

In making pH determinations of coated film, a definite areaof film is cut into pieces and soaked in freshly distilled waterand the water tested against a similar quantity of freshly dis-tilled water as a control. Colorimetric tests can be made withan indicator having a range somewhere between 5 and 8 pH;or if necessary, two tests may be made with indicators each ofhalf the range. For tests of dyed or colored materials, anelectrometric apparatus such as the Beckrnan is necessary; inEngland portable apparatus is made for pH determinations by


the Cambridge' Instrument Co. Owing to the rapidity withwhich even fre hly distilled water decreases in pH in the at-mosphere of big manufacturing towns and cities, the water usedshould preferably be freshly distilled a few minutes before thetest is made.

KEEPI G TESTS. - An estimate of the probable keeping qual-ity of sensitized materials is of importance in laboratory workand is essentially part of the ordinary routine in commercialproduction. It might be imazined that the faster the emulsionis, the poorer would be its keeping qualities. This, however,is not always the case, some slow emulsions being troublesomein this respect. Thinly coated plates are liable to go off at theedges (to fog round the edge on development withou t exposure) ,and any products which have given evidence of slight fog ontesting after being emulsioned must always be regarded withsuspicion. There is a tendency for any emulsion that has beenpushed too far for speed, or over-heated, to give veil, but refer-ence to this will be made again in Chapter XIV. The veil, orfog, must be deducted from all readings in sensilometric work,and for a practical test the developer that is recommended foruse with the material should be used in making that test.

While a 'slow positive or process emulsion should not, onnormal development without exposure, give a higher fog read-ing than 0.02, that on an ultra-rapid emulsion may legitimatelybe of the order of 0.1. On keeping for any length of time, thefog reading usually increases, especially if the material is storedunder warm or humid conditions, and in order to get an estimateof the life of the product, it is common practice to subject itto an oven test. The material, wrapped in the packing whichit is intended to use, is kept in a thermostatically controlled hotair or water oven at a temperature of IOSo or IIOo F. for aboutten days. It will then have developed approximately the sameamount of fog as would be anticipated from, say, six months'


storage in the average dealer's stockroom. The humidity of theoven is an arbitrary matter, but SS to 65 relative humidity issuggested as being fair.

When making fog tests, it is customary to pack t~e materialin two halves, one being kept in the oven, the other being keptat room temperature, under the best possible conditions. Theinitial laboratory tests will, of course, also have been made forreference. After the period of ovening or " incubation," thecontrol and the oven samples are developed together in totaldarkness and fixed, and the fog readings compared. A sen-sitometric test will also reveal any loss or gain in speed or altera-tion in the development factor.

While at first sight keeping tests seem simple and straight-forward enough, they are complicated by the fact that whenmaterial is stored in an oven, the sides, top and bottom ofthe inside of the package do not necessarily experience the sameconditions. An elaboration of the tests is to place the packagesin a cage which rotates in the center of the oven; many othersuggestions have been made. Highly red-sensitive and infraredemulsions, owing to their natural reaction to the longer wave-lengths, are apt to develop fog in warm climates quicker thancolor- blind varieties.

The oven test is invaluable in checking up on wrapping ma-terials. 0 wrapping papers, nor cardboard boxes, should beaccepted for the packing department's use unless and until theyhave been demonstrated by an oven test to be harmless in theway of fog production. In the case of roll films which are mer-chandised in aluminum containers, sealed or unsealed, the oventest should be made with the spooled films in these, so thatabsolutely practical conditions are obtained. Materials des-tined for tropical work or expeditions which are to be suppliedin soldered tin boxes should again be submitted to an oven testso packed, the reason being that at 1100 F., or thereabout, a


certain amount o.f moisture may be evolved in which the sen-sitized materials may show some alteration.

Chapter References

1. O. Bloch, Photo Lourn., 55,1915.2. T. T. Baker, Photo Journ., 65, p. 60,1925.3· A. H. Nietz, Theory of Development, p. xxx.4· L. P. Clerc, Photography, Theory and Practice, p. 136.5· F. F. Renwick, Trans. Soc. Mot, Pic., Eng., 1924, p. 69.6. G. Z. Dimitroff, Harvard Coll. Obs. Circ., 4.30, 1938.7· Capstaff and Purdy, Trans, S. M. P. E., 9, 607, 1927.8. W. B. Ferguson, Bru, J. Phot, p. 258, 1926.9· F. C. Toy and S. O. Rawling, Comm. No. 40, B. P. R. A. Labs.

10. L. A. Jones, 1. Franklin Inst., 1926-7.11. F. F. Renwick, Photo J., 77, Jan. 1937.

CHAPTERXIII

VARIOUS METALLIC PROCESSESCarbon and Carbro Tissue - Gum-bichromate - Iron Printing Processes- Ferroprussiate - Cyanotype - Kallitype - Platinotype and Palladio-type - Diazotype Papers - Bleach-out Color Processes

THIS method of printing depends upon the action of lightupon a dichromate in the presence of orzanic matter. The

response to light was shown by Cartwright 1 to be most activein the region between 350 mu and 420 mu. Attempts have beenmade by Ronald Trist and others to sensitize dichromated gel-atin or fish glue to green and red rays by color-sensitizing, with-out particular success. On exposure to light in the colloidalmedium, the dichromate is converted into a brown chrome-chromate consisting of uncertain proportions of chromic acidand the green oxide of chromium:

According to Eder, the insoluble image consists of gelatin incombination with chromium oxide, while Lumiere and Seyewetzfound that under normal conditions the proportion of chromiumoxide combined with the gelatin is considerably higher thanthat which combines with gelatin treated with chrome alum inthe ordinary way. The exposed dichromated gelatin swellsmuch less in cold water, and is insoluble in hot water. So theprinted image, on washing with hot water, consists of insolubil-ized gelatin with the unexposed soluble gelatin removed.

It was found by Abney that the insolubilizinz action of lightcontinued after a short preliminary exposure, indicating theformation by the action of light of a photo-catalyst," whichcontinues the tanning action by reaction with the excess of di-c,hromate and colloid. Unfortunately, sensitized" tissue," as

VARIOUS METALLIC PROCESSES 237

paper coated with the gelatin and dichromate is termed, rapidlybecomes insoluble on keeping in the dark, and hence it is custom-ary, in fact necessary, in manufacturing practice to supply thegelatin-coated paper which merely has the necessary dye or pig-ment incorporated with it. Some firms supply sensitized tissue,but it will keep in good condition for only a very limited time.

Owing to the important property, possessed by dichromatedgelatin, of giving an almost perfect scale of reproduction, thecarbon process and its ally, Carbro, giye unsurpassed three-color prints from a correctly balanced set of separation nega-tives. The carbon process is largely responsible for the ex-cellent tone reproduction in photogravure, and its use for thisprocess makes it an important item in manufactured products.

Carbon tissue is paper coated with a mixture of gelatin andpigment, which the user sensitizes shortly before use by immer-sion in a solution of potassium or ammonium dichromate whichmay be varied from one per cent for weak negatives to six percent for vigorous negatives. In making the tissue, a moderatelyhard gelatin which does not swell excessively should be selected,as soft gelatins have a tendency to wash out too easily in de-velopment. Heinrich's hard emulsion gelatin or a comparableAmerican variety can be recommended. The raw paper se-lected should not be too highly sized, but well calendered. Forexperimental work, a plain smooth paper such as is used aswallpaper for coverinz ceilings will be found very suitable.

An advantage of the carbon process is, of course, that anysuitable pigment can be used which is fast to light, so that printscan be made in any color. This is of especial importance innatural color photography, since coloring matter can be usedwhich complies with theoretical requirements as regards spectralcomposition. Thus yellow, magenta and blue-green papers areprobably the types most demanded at the present time. Forsmall-scale work the moist water colors supplied by art dealers


in collapsible tubes make good media, although certain pig-ments may be found to react with the dichromate, causing in-solubilization without exposure.

For commercial mixings, these formulas are suggested:Sepia Burnt sienna 90 parts

Lampblack 10 parts

Warm black Chinese white " 25 partsVandykc brown 40 partsCarmine 40 parts

Brown-red Indigo red 40 partChinese white 35 partsAlizarin red 25 parts

Prussian blue and indigo are largely used also. The pigmentsare weighed out and mixed with an equal weight of water, andfive per cent of the total weight of glycerin is added. The mix-ture is well ground in a color mill, and then added to twentytimes its weight of the following:

Gelatin .Water .5 per cent alcoholic solution of phenol .Soft soap : .Sugar .

125 g1000 cc

50 cc20 g25 g

After thorough mixing, the bulk is filtered at a temperature oflOSo F. (400 C.), and coated on a paper-coating machine, pref-erably by kissing roller.

A formula for black tissue given by Clerc 3 is:Gelatin 500 to 800 gCut sugar 100 to 200 gSalicylic acid 2.5 gVegetable black. . . . .. .. . . . . . . .. . . . . . . . . . . 100 gCarmine 10 gIndigo " 5 gGlucose syrup 100 ccWater 2500 cc


For preparing a' ready-sensitized tissue, fifty grams of potas-sium dichromate and ten grams of potassium citrate would beadded, first dissolved in 250 cc of water, to which sufficientstronger ammonia water has been added to change the colorfrom orange to pale lemon-yellow. Such tissue must be usedwithin three or four days.

The sensitized tissue is printed under the negative and soakedin cold water, then brought into contact (under the water) witha sheet of either temporary or perma.nent transfer paper. Ifsingle transfer paper is used, the image .naturally becomes re-versed in the process. Such paper may be coated with:

Medium viscosity gelatin 50 gWater 1000 cc5 per cent chrome alum solution. . . . . . . . . . . . . 10 cc

The addition of 250 cc of a twenty per cent alcoholic solutionof shellac will provide a mat transfer paper. The support paperand printed tissue, when withdrawn from the cold water, aresqueegeed firmly together and allowed fifteen to twenty minutesbetween blotting. boards before they are immersed in warmwater, pulled carefully apart, and the unexposed coating washedaway from the transferred image.

The image may be developed out on a temporary support,which may be paper coated with shellac and rolled, or somematerial such as celluloid, aluminum sheet, sheet indiarubber,etc., first prepared with a waxing medium. The temporarilytransferred image is re-transferred to a final support, which maybe a paper coated with the formula above given, but the weightof paper base should be heavier.

Plain slightly subbed nitrate or acetate base coated with aseven or eight per cent solution of gelatin containing no chromealum may be sensitized with dichromate and used for makingthree-color prints from separation negatives. A slow bromide


emulsion with no hardening agent may be coated on film basefor the same purpose. The silver salt plays no part in theprocess, but makes the image much more plainly visible inprinting, and more controllable. Two minutes' iIU:l11ersionina three per cent solution of potassium or ammonium dichromatewill suffice for sensitizing, and this can be done in yellow light.The material is printed through. the base (the gelatin or emul-sion side away from the film of the negative), and after exposurethe image is developed in hot water of about 1600 F. (720 C.).Washing-out must not be too vigorous, as a very thin film ofgelatin should remain over the faintest highlights. If a bromideemulsion has been used, the film must of course be fixed afterwashing out, and finally well rinsed. Such gelatin relief imagescan be stained up with appropriate dyes and mounted in registeras three-color transparencies.

GUM-BICHROMATE.- While the image from a negativeprinted upon carbon tissue must be transferred to another baseto be developed, a mixture of coloring matter with gum arabic,sensitized with dichromate, can be applied to paper, and thedry paper exposed under a negative in daylight and developedwith cold water without any transfer. The unexposed gum-pigment remains completely soluble and washes away, whilethe exposed parts remain and provide the image. The gum-bichromate process, as it is familiarly known, is very suitablefor prints of bold subjects, especially in the larger sizes, butdoes not lend itself to the reproduction of fine detail. The car-bon process, on the other hand, is capable of yielding the finestdetail, as is evidenced by its application to photogravure, wherehalftone screens of 150 to 175 lines to the inch are used.

Gum arabic is best crushed into small pieces and suspendedin a muslin bag in distilled water; it takes some time to dis-solve, but the dirt ordinarily present in the gum will remainbehind in the bag. The gum should be dissolved in about one

VARIOUS METALLIC PROCESSES

and one-half times its weight of water. To one hundred cubiccentimeters of such mucilage is added:

Potassium dichromate 10 gWater to make 100 cc

To such a mixture sufficient pigment is added to give a coating- when applied with a brush to paper - of a density whichwill allow the finger to be seen through the paper when examinedby transmitted light. In other words, the tint applied to thepaper should be considerably less than that required to formthe shadows of a print. Rouge (iron oxide), lampblack, burntsienna, Prussian blue, indigo, and alizarin red, being inert tothe chemicals employed, are the best pigments to use. Anotherformula for gum-bichromate is the following:

Gum arable solution I: I . . . . . . . . . . . . . . . 50 ccAmmonium dichromate, 10 per cent solution . .. 100 ccPigment " 25 g

The prints are best developed by floating face down on coldwater, and gently washing out until the whites are clear. Fiveminutes in a five per cent solution of potassium alum beforethe final wash will render the film hard.

IRON PRINTINGPROCESSES.- Iron forms two principal typesof salts, ferric and ferrous. The ferric alts are combined withhalf as much again of the acid radical as the correspondingferrous salts. The action of light upon the ferric salt of anorganic acid wa early noted, and is to reduce it to the ferrousstate; an iron process was based on this phenomenon in r842by Herschel, who used the ferrous compound to react with ferri-cyanide and form a permanent image. Ferric citrate and ferricoxalate are reduced respectively to ferrous citrate and ferrousoxalate; similarly potassium ferricyanide, K3Fe (C ) u, is re-duced to ferrocyanide, K1Fe (C ) 6' These ferrous salts formmany highly colored pigments by reaction with salts of other


metals with which they form insoluble ferrocyanides best knownof which is Turnbull's blue. If paper sensitized with a ferricsalt is exposed under a negative and developed with a suitablere~~ent, a permanent colored image will result; tRe most fa-miliar example is the engineer's blueprint; this is made onpap:r se~sitized with a mixture of a ferric salt and potassiumferr.lcyal1l~e. Printed under a design or translucent drawinguntil the lines appear of a light brown color, a mere wash incold water will effect the reaction and leave a deep blue imageof ferrous ferrocyanide.

This process, known as ferroprussiate or cyanotype, is usedon a very large scale, although it has experienced some com-petition in r~cent years from the vapor-developed diazotypepapers to which reference will be made later. The sensitizinzsolu~ion can be applied in strong yellow light, and any goodq.uahty, pure paper can be treated. It offers the simplest pos-I~le .method of printing photographs, and can be applied to

pnntmg on cotton or linen fabrics; these, if printed and de-veloped in water in the usual way, can be toned to a beautifulviolet or purple by treatment with a boiling solution of alizarin.

The paper is usually prepared with ferric ammonium citratewhich may be obtained in the form of brown or green scales:The green scales have the average composition of (C

6H.O.),

Fez (NH.) 3, while the brown scales are (C6H501) 2H:F~2-(OH)3(NH.),3HO. Valenta found that the green compoundga:e very much quicker printing with brighter blue images. Areliable formula for a sensitizing solution is:

A. Potassium ferricyanide .Water .

B. Green ammonium ferric citrate .Water

40 g250 cc95 g

................................. 250 cc

If the brown citrate is used (which is claimed to zive ratherbetter keeping quality for manufactured Ierroprussiate paper):


the quantity may be increased to I ID g, and the water in Bincreased from 250 to 500 cc. The latter sensitizer also givesgreater contrast and is useful for copying plans, tracings, etc.,while the first formula would be preferred for photographicprints. The two solutions are mixed, filtered, and used cold.It is very important that the mixture is orange-brown coloredand not green. It is advisable to use large crystals of potassiumferricyanide and before weighing them to rinse them with a littledistilled water; this will remove any yellow ferric salt withwhich they may be surface-contaminated. The salt should bedried on filter paper after rinsing, before weighing. If stocksolutions are made up, a trace of sodium formate may be addedto prevent the formation of mold.

Another formula, suitable for coating larger quantities, pro-vides for a mixture of stock solutions:

25 per cent solution of ferric ammonium citrate 400 cc15 per cent solution of potassium ferricyanide ISO ccIO per cent solution of citric acid 300 cc

The ferric ammonium citrate should be freshly made, and papersensitized with the formula will keep in good condition for twoto three months if stored in a dry place. The addition of phos-phoric acid to the sensitizing solution has been suggested tostabilize the lines and the blue background.

Forcommercial manufacture, sensitizing by dipping will befound suitable practice. In the laboratory, the solution canbe applied with a wad of cotton about the size of a golf balltied with thread to the end of a glass rod. A wide soft brushmay also be used. The paper is laid out on a flat board orpiece of thick cardboard and held down at the four cornersby pieces of scotch tape. It is then brushed over as evenly aspossible. .The board is best inclined to the table at an angleof about thirty-five degrees. In machine coating, an excess of

244 PHOTOGRAPHIC E 1ULSION TECH IQUE

solution is applied, and the paper wiped by a doctor just beyondthe coating roller, leaving the desired thickness evenly applied.Commercial coating weight is about 60 to 65 g of iron saltsper 100 square meters - roughly one grain to the ~quare foot.The coated paper should be dried in total darkness.

Printing is best done in sunlight, though arc or mercury vaporis satisfactory. Ferroprussiate paper gives a deep blue imageon a white ground, and of course gives a " negative" of a linedrawing, white lines on a blue background. If, on the otherhand, blue lines are wanted on a white background, this effectcan be obtained by coating with a solution of gum arabic sen-sitized with ferric salts, and after printing, developing it witha ten or twenty per cent solution of potassium ferrocyanide, onwhich it is usually floated. An alternative method, given byPizzighelli,» admits of simple development by washing in water;the sensitizing solution is as follows:

20 per cent solution of gum arabic 200 cc50 per cent solution of ferric ammonium citrate . . . . . . . .. 80 cc50 per cent solution of ferric chloride. . . . . . . . . . . . . . . . .. 50 cc

On mixing the above, a very viscous solution is produced, butafter standing for a few hours it disperses and becomes thinenough to be applied with a brush or wad of cotton. As thispaper is considerably more sensitive to light, it should be madein yellow safelight and dried in complete darkness.

KALLITYPEPAPER..- This is printing paper sensitized withferric oxalate, which is reduced by the action of light to theferrous state; the strong reducing action of the ferrous salt isthen used to develop a silver image, but, owing to its insolu-bility, the presence of an organic salt such as potassium oxalateis required to act as a solvent. The printed paper may be putinto a bath containing silver nitrate, or the silver - or part ofit - may be incorporated in the sensitive coating. The simplest


means of sensitizing is to use a composite solution such as thefollowing:

Ferric oxalate .Potassium oxalate (neutral) .Silver nitrate .Distilled water .

22·5 g5·5 g5·5 g

100 cc

A smooth, thin drawing or water color paper or plain raw photo-graphic paper is first sized with a paste made by mixing threegrams of starch with a little water until smooth, and then stir-ring while sufficient boiling water is added to bring the bulkup to two hundred cubic centimeters. This solution is boiledfor five minutes, cooled and poured into a beaker or crock.When cold, a skin will be found to have formed on it. This isremoved, and the liquor is strained through a piece of washedmuslin into a clean dish. The paper is best coated by floating,unless a small coating machine such as has been described else-where is available.

When dry, the paper is printed in daylight until the shadowsare just visible, and is then developed with an alkaline solu-tion of Rochelle salt. A basic formula is:

Warm water 500 ccRochelle salt ~ . . . . . . . . . . . . . . . .. 30 gBorax 15 to 25 g10 per cent solution o'f potassium dichromate. . 5 cc

The full quantity of borax gives black tones, a smaller amountgiving warmer tones.

When printing from weak negatives, a slight increase in theproportion of dichromate may be used. If the contrast is toohigh, the print can be removed from the developer and trans-ferred to a separate bath containing no dichromate. When de-veloped to completion, the print is fixed in a two and one-halfper cent solution of plain hypo rendered just alkaline by theaddition of a few drops of ammonia. An attractive feature of


kallitype is that the image. can be toned afterwards with a goldbath, such as is used for printing-out paper, or with platinumor palladium.

PLATINOTYPE.- The successful results obtained .,vith kalli-type led W. Willis in 1878 to substitute platinum for silver,and so to produce what still remains the finest printing mediumavailable to photographers - platinotype. Prints made by thisprocess have an exquisite range of gradation. The image isabsolutely permanent. If we take the tone range of a contrastychloride paper as I: la and of a good bromide paper as I: 50,then we may justly say that the gradation of platinotype isI: 100.

The paper itself, needless to say, must be of pure quality,and especially free from iron spots or specks. It must notbe sized with an animal (glue) size, though a first coating witha one per cent solution of the purest emulsion gelatin may beapplied. A three per cent solution of arrowroot, boiled andfiltered, is better. The paper may be immersed in the solutionfor five to twenty minutes, according to the surface, and thendried slowly in a cool atmosphere.

For sensitizing, three stock solutions are prepared:

A. Potassium platinum chloride. . .. . .. .. .. .. gWater 6 cc

B. Ferrous oxalate .. . . . . . . . . . . . . . . . . . . . . . . . gWater 4.5 cc

C. Ferrous oxalate .. . . . . . . . . . . . . . . . . . . . . . .. 10 gPotassium chlorate . . . . . . . . .. 0.2 gWater 45 cc

For sensitizing, mix 5 cc of A and 3 cc of B; add 2 cc of C and4 cc of water. For contrasty negatives, the C solution may beomitted and equal parts of A and B only used. The sheets ofsized paper, cut to size, should be laid on a sheet of clean glassor in the bottom of a porcelain dish, and the sensitizing mixture


applied as evenly as possible with a soft camel's hair brush.This may be done in Mazda light.

Paper prepared as above described is printed in sunlight untilthe dense shadows are of a light orange-brown color; no detailsshould be seen in the extreme highlights. The print is developedwith a hot solution, 1000 to 1800 F., of:

Potassium oxalate (neutral) 300 ccOxalic acid 5 gWater to make -. 1000 cc

Alternatively, cold development may be used if the sensitizingbath be somewhat modified as follows:

A. Solution as in above formula.B. Ferric oxalate 10 g .

Lead oxalate 0.5 gWater 45 cc

C. Potassium dichromate. . . . . . . . . . . . . . . . . . . gWater 10 cc

4 cc of A is mixed with 6 cc of B. To this mixture is addedla drops of C mixed with 3 cc of distilled water.

A printing-out platinum paper for giving a visible image canbe made as below:

A solution as above 4 cc50 per cent solution of sodium ferric oxalate .. 6 cc50 per cent gum arabic solution 4 ccC solution as above 5 drops

Paper sensitized by brushing it over with this solution (afterthe usual sizing) is printed fully out, and then finished off bywashing or placing between wet filter papers. All platinumpapers should be given several final rinses in water acidulatedwith one or two per cent of hydrochloric acid.

Owing to the difficulty of making entirely satisfactory platin-otype papers in the laboratory, Clerc recommends 5 the easier

\


procedure of sensitizing the paper with ferric salts and usingthe platinum compounds in the developing solution. The siz-ing solution recommended is:

Gelatin 8 gWater 1000 ccWhen melted and dissolved, add

Potassium alum 2 g

The paper is floated on this solution, used hot, and the uppersurface (hereafter the back side) should be marked with apencil. The sized paper is brushed evenly with the sensitizingbath below:

Ferric oxalate .Oxalic acid .Lead oxalate .Hot distilled water to make .

25 g2 gI g

100 cc

The lead oxalate may be made by mixing equal volumes of aten per cent solution of lead acetate and a four. per cent solu-tion of oxalic acid, filtering and washing the white pasty pre-cipitate left in the filter paper several times with distilled water.The image on printing is much less visible than in the case ofthe papers previously described, and can be judged only byexperience or by comparison with some form of print-out acti-nometer. The developing solution, containing the platinumsalts, is made thus:

Stock SolutionPotassium oxalate (neutral) 200 gDi-sodium phosphate 50 gWater to make 1000 cc

To 20 CC of this stock solution is added 2 cc of a ten per centsolution of potassium chloroplatinite. Development is donewith a flat camel's hair brush, moved over the image" with aquick light movement, dipping it in the developer for each

VARIOUS METALLIC PROCESSES 249stroke so as to insure equal and uniform action of the solution."When evenly coated with developer, it is left until completelydeveloped, fixed in weak hydrochloric acid, and washed.

Owing to the high cost of platinum, palladium has been usedon an increasing scale. Palladium salts may be substituted forplatinum in many of the above formulas, particularly in de-veloping formulas where the paper itself is sensitized only withthe iron salts.

PRI! TING WITH COPPERSALTS.- A formula for sensitizing.with copper salts has been suggested by Thiebaut 6 for use onpaper previously substratumed with gelatin:

Potassium dichromate 90 g

Copper sulphate . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 gWater to make IOOO cc

The paper is exposed in daylight until the highlight detail isclearly visible, and is then developed in a five per cent solutionof anilin hydrochloride acidulated with sulphuric acid. Greenimages are given by this process.

URANIUMPAPER.:- This is prepared by the application tounsized paper of:

Uranyl nitrate 16 gWater to make " " . . . . . 100 cc

The suggestion has been made of adding four grams of coppersulphate to this solution. It is printed in daylight, preferablyunder a vigorous negative, and developed with a five to eightper cent solution of potassium ferricyanide. Warm sepia orbrown images are given by the process, and various metallictoners can be applied.

DIAZOTYPEPROCESSES.- Methods of photographic printinghave been successfully based on properties of certain aromaticdiazo compounds which become bleached on exposure to light.The unexposed parts remain unaltered, and on development

250 PHOTOGRAPHIC EMULSIO TECH IQ E

with an alkali, or with ammonia vapor, an image remains con-sisting of a permanent azo dye, thus giving a positive imagefrom a positive original.

In the Feertype process," on the other hand, a liye is formedin the exposed portions of a diazo compound which can be de-veloped in acidulated water. A sensitizing formula of this na-ture is as follows:

Iso-sodium toluol diazo sulphonate. . . . . . . . . . 25 gBeta naphthol 25 gSodium hydroxide 8 gWater 1000 cc

In Ozalid paper," Koegel produced positive from positiveprints in purple lines on a slightly colored background. Thepaper was impregnated with a solution of a diazo-anhydrideand resorcin equilibrized with tartaric acid. The diazo-anhydride is decomposed by light, but the unexposed materialis converted to an intense purple-brown on exposure to am-monia vapor. The fact that the paper is developed dry elim-inates distortion. Under the name of Ozafilm, diazo compoundshave been introduced into viscose for sub-standard motion pic-ture films, having a thickness of only a thousandth of an inch,so that the bulk of the reel can be greatly reduced.

More recently, chinonazide has been used as a sensitizer; thiscompound is diazotized para-aminophenol, which under the ac-tion of light becomes converted into hydroquinone. If thepaper, after printing, is treated with a weak solution of silvernitrate, a latent image is formed which can be physically de-veloped with a metol-sulphite-silver bath.

On exposure to light a diazo compound loses its faculty ofcoupling with a coupling component, being probably decom-posed into a phenol. In those parts where the light has notacted, and the diazo retains its original composition, it is ableto form an azo dyestuff by the combined action of the coupling

VARIOUS METALLIC PROCESSES

component and an alkali." In the simplest case, the couplingcomponent can be mixed with the diazo compound, when, afterprinting, development can be carried out with ammonia vapor.An alkali can, on the other hand, be mixed with the azo couplingcomponent. An example of such a sensitizing solution is:

p-arnino-dimethyl anilin hydrochloride,diazotized with nitrous acid .

Tartaric acid .Barium chloride ',' .Water to make .

20 parts2 parts

100 parts1000 parts

Paper sensitized with the above is printed and developed witha weak alkaline solution of ~-oxynaphthoic acid anilide. Thisgives a black image on a white background.

Another formula is as follows:para-amido-ethyl anilin hydrochloride,

diazotized with nitrous acid .Potassium citrate .Barium chloride .Water to make .

20 parts0.5 part

100 parts1000 parts

This gives a dark brown image on development with an alkalinesolution of ~-naphthol. The barium chloride prevents bleedingof the developed azo dyestuff; the use of a catalytically actingsubstance called an anti-oxygene such as pyrogallol or meta-phenylendiamin has been suggested by F. van der Grinten.

PRINTING-OUTNATURALCaLOR PAPERS.- Very little infor-mation is available as to the preparation of sensitive coatings offugiti~e dyes with which direct printing-out effects can be ob-tained from natural color transparencies, yet this is a field forexperiment which may well see immense developments. It wasnoted by Grothuss that certain dyes were destroyed by the lightradiations which they absorbed; in other words, a fugitive ma-genta dye would be bleached by green light, a blue dye by yel-low light, and so on. A black mixture of blue-green, magenta,


and yellow dyes applied ·to paper and printed out under a colortransparency would yield a color print.. Bleach-out color papers were made by euhauss, Worel, andothers, but they were too insensitive to be of prastical value;]. H. Smith, however, succeeded in producing a black bleach-out paper which gave a fairly creditable print from an Auto-chrome in one to three hours' exposure to strong sunshine. Theauthor at one time experimented with Dr. Smith to employ arclight printing, but the speed did not justify the continuationof the work. Also the bleach-out images, though fixed in vari-ous ways, were not by any means permanent.

An example of possible advance in this direction may be seenin the very marked increase in light-sensitivity 'of methyleneblue if used in the presence of certain thiourea derivatives.Three dyes of the subtractive colors sensitized in a similar wayand balanced so as to bleach out at equal rates after allowancehad been made for the relative transmissions of the primarycolors of the transparency, would yield a natural calor printingmedium, provided the sensitizers could be destroyed after print-ing, and the remaining dyestuffs rendered reasonably permanent.

Indirect processes employing more light-sensitive coatingswhich would, under the influence of light, react in turn withthe dyes and destroy them, figure among present attacks onthe problem.

Chooter References1. H. Mills Cartwright, Photo J., 1923.2. W. de W. Abney, Instruction in Photography, 1900.

3· L. P. Clerc, Photography, Theory and Practice, p. 417.4· J. Pizzighelli, Anthratypie et Cyanotypie.5· Photography, Theory and Practice, p. 410.6. A. Thiebaut, La Pbotographie, p. 277, 1908.7· A. Feer, G.P. 53,455,1889.8. Koegel and Steigmann, Zeit. wiss. Phot; 24, 17I, 1926.9· E.P. 28I-604 of 1937.

CHAPTER XIVEXTREME-SPEED EMULSIO S

Super- and Hyper-scnsitizers - Sulphur Compounds in Gelatin - Anti-fogging Agents - Speed Characteristics

I considering th~ faster emulsio~s whichhave b~en r~centlyintroduced, one IS tempted to raise the old question: What

is speed?" It has already been seen in Chapter XII that ifthe toe of the characteristic curve of a fast emulsion is suffi-ciently straight to permit of the whole tone range being recordedon it, that is, provided that such density range as it offers willsuffice for the work in question, a very much higher effectivespeed is obtained than with the same emulsion developed toa higher gamma and the upper straight portion of the curveused. Similarly if the free bromide be removed from the emul-sion, the whole curve may rise from AB to A'B' (Fig. 78),

K Kthe speed being thereby raised from _ to __ . If thereforeOC OA'

the need for free bromide could be dispensed with by the useof an anti-fogging agent, an immediate rise in speed could beobtained. The removal of the bromide is 'substantially whathappens when a plate or film is " hyper-sensitized " by bathing.

The line of thought which actuated many emulsion-makersin the stage intermediate between the first really fast ortho-chromatic and· panchromatic emulsions and the more recentlyintroduced extreme-speed variety may be gathered from a com-parison of the two following formulas, one of which was pub-lished as an indication of how the other basic one might be" pushed ,~for greater sensitivity:


A. Potassium bromide .Potassium iodide .Gelatin .Water .Erythrosin I: IOOO ....•....•..•....

B. Silver nitra te '" .Water .Redissolve.

Fast

120 g3 g

50 g1200 CC

cc

150 g600 cc

30° C.

rapid

Temperature of reacting solutions ..

Emulsifying time

Ripening time '" . . . . . . . • • . . . . . . 25 to 40minutes

Ripening temperature , .Bulk gelatin .

40° C.120 g

Extremespeed132 g

4·5 g30 g

1000 cc50 cc

150 g500 cc

40° C.

slow, taking6 minutes

30 to Sominutes50° C.

140 g

The gelatin is soaked for half an hour, drained and remelted,and added at 45° C. in each case; after ten minutes' stirring,the emulsions are set in ice water. The fast emulsion is washednext day, the extreme speed one, after keeping at 3° to 8° C.for two to three days. In the case of the extreme speed emul-sion, further gelatin may be added again after washing; theemulsion is then warmed to between 40° and 42 ° C. and stoodin a room at about 22° C. for two to three hours.

It will be recognized that this difference really amounts to agentle push all the way around. With a well- elected gelatin,the speed may be raised from say 650 to 1500 Hand D in thismanner, but the author's experience of forcing an emulsion onthese lines is that it is liable to yield variable results, and thatsome of the best emulsions so prepared, which are perfectlyclean at the time of making, turn out disappointing from thekeeping standpoint, developing fog and steadily losing speed.

A good deal of progress in the direction of speed has been

EXTREME-SPEED EMULSIO S 255

made as the result of more critical testing of gelatins. Oneexample of this, of outstanding importance, is the fact that bygiving a sufficiently long digestion, what had hitherto beenregarded as a " slow" gelatin may be made to give the speedof a " fast" sample. This digestion may have to be very pro-tracted, lasting for many hours instead of from twenty minutesto an hour or so. But the time-factor appears to be of vastlymore importance than had been previously recognized.

IIIA LOG £

o

FIG. 78

As stated elsewhere, the great advance in panchromatizing-dye chemistry has given a fictitious but perfectly practical in-crease in speed to some calor-sensitive negative emulsions, theresponse to light of the longer wavelengths making an important

. addition to their sum total of response. The discovery thatcertain dyes act as super- or hyper-sensitizers, greatly increas-ing the sensitivity induced by another dye the calor-sensitizingregion of which they do not affect, has given another jump in


speed, of important practical value, although the phenomenonlooks at first sight a littie like perpetual motion.

The great rapidity of recently introduced emulsions can beascribed largely to the use of stabilizers, the addition of which••to an emulsion in the making renders it possible to force thespeed or to add" sensitizers " without pushing a proportion ofthe grains too far and rendering them developable without ex-posure as "fog." An instance is the use, suggested by Ken-dall,' of a 2-mercaptan-4-hydroxy-pyrimidene, or one of its al-kyl, aralkyl or aryl substitution derivatives.

We must to some extent associate color-sensitizing with hyper-sensitizing; the carbocyanins and similar products in generaluse, as we have seen, are extremely sensitive to light, but be-come stable to light when treated with certain derivatives ofthiourea; yet thiourea or thiocarbamide, CS(NH2)2' by itself,is one of the most effective emulsion sensitizers. The choiceof sensitizer must obviously be coordinated with that of thecolor-sensitizer; indeed, one may be largely dependent on theother.

The sensitizing property of allyl isothiocyanate, H5C:l·NCS,which as we know was isolated from gelatin by Sheppard, andshown to be chiefly responsible for emulsion" speed," cannotbe used ad libitum; in other words we cannot, by dou bling thequantity used, obtain double the speed. Indeed, for many yearsafter the publication of the function of allyl isothiocyanate, thegeneral speed of plates and films went up very little, and suchadvances as were made were not directly due to its exploita-tion. Allyl isothiocyanate is the essential oil of mustard, andcan be obtained by macerating black mustard seeds with water,after first removing the fixed oil, then distilling. When mixedwith ammonia and subjected to ammonia gas; it yields allylthiourea or thiosinamin, NH2NH(CaH5)CS; this is a weakbase, soluble in water, alcohol and ether.

E-SPEED EMULSIO. IS 257

A test for these sulphur compounds in gelatin has been givenby Luther," who suggests the use of a solution of twenty totwenty-five per cent of caustic soda (sticks) in 100 cc of distilledwater, in which 3 to 4 g of pure lead nitrate has been dissolved.One volume of this alkaline lead nitrate solution is mixed withone volume of a fifteen to twenty per cent solution of the gel-atin to be tested, and placed in a test-tube in a boiling waterbath. A good quality gelatin (containing the most useful pro-portion of sulphur compounds) will show a brown colorationafter about half an hour, which will remain visible after onehour.

Selenium compounds have been largely investigated, for theiruses both as sensitizers and as anti-fogging agents. Thus ad-dition of an c-thione or cx- elenone of a heterocyc1ic nitrogencompound such as thiazole, quinoline, pyridine, etc., has beensuggested for speed-sensitizing, while the ·incorporation of asulphinic or seleninic acid or soluble salt of either has beenused as a stabilizer against fogging." Amino-purines have beensuggested for the same purpose, that is, stabilization. A com-pound of the purine series has also been employed, containinga hydrogen atom which is replaceable by silver attached to anitrogen atom, when promoting ripening by means of a sulphurcompound.

While a very large number of patents have been grantedsince about 1933 for the protection of color-sensitizers, colorhyper- or super-sensitizers, and emulsion sensitizers and stabi-lizers or anti-fogging agents, there must always remain a clearfield for experimentation and further research. The additionof any extreme-speed sensitizer must first be tried out with afew pre-selected gelatins, among which an iso-electric samplemay well be present; sensitizers which promise success mustthen be tested in conjunction with anti-fogging or fog-retardingcompounds, until a suitable balance is obtained. The sensitizers


mayor may not be intimately associated with a color-sensitizerin the case of panchromatic emulsions. The final emulsion maybe further assisted by appropriate buffering, which may wellbe made to function in the undercoat of a slower, saable emul-sion in the case of double-coated film stock.

The Hand D speed of modern emulsions, while it increaseswith prolonged development, appears according to Trivelli andSmith 4 to reach a breaking point long before the gamma hasreached its maximum value; beyond this point the speed di-minishes. This is an important point, as although the observa-tion applies to some extent to older-type emulsions it emphasizesthe necessity for careful sensitometric testing of a super- peedemulsion in order to determine the conditions under which itsoptimum speed can be realized.

Chapter Rejerences

1. J. D. KendalI, E.P. 5078, 15819, 1935.2. R. Luther, Photo Lndustrie, O. 20, 1927.3· G.P. 2I478, 21479, 21480, 1934.4. Trivelli and Smith, Photo I., 80, p. 384, 1940.

A

Abney, W. de W., x, !IS, 236Acetate film base, 17SAgar-agar, 2Azfa color plate, 81Air conditioning, IS I

Alcohol, 3$, 49, I53Allyl isothiocyanate, xi, 256

thiourea, 256Ammonia, 8, 32, 67, 69,78,93Arago, IArlex, 173, 184, 185Auramin, 107Autochrome plate, 81, 82

B

Baker, T. T., 83, 97, 131, 132, 215"227,252 I

Baryta coating, 157Bathing of plates and films, lIOBayley, R. Child, 25Beckerel, 23Bipack emulsions, 121Bleach-out papers, 25IBlending emulsions, 77Bloch, 0., 203Bogue, R. H., 21Boiled emulsions, 6Bolton and Sayee, 2Breaking up emulsions, 70Buffering of emul ion; 72Bulk gelatin, 22Bunsen and Roscoe, I9

C

Cadett, J arnes, 149Capstaff density meter, 218Carbocyanins, rr6

INDEX

Carbon printing, 237tissue, 238

Carbro process materials, 237Carroll and Hubbard, 13, 16, 17, 72Cartwright, H. M., 236Cascade spreader, 149Celluloid, 175Cellulose acetate, 2, 175

nitrate, 175Characteristic curves, 204, 222Charlesby, A., 129Charrion and Vallette, 175Chemicals, types of, 3 I

Chloride emulsions, 94, 99Chloro-bromide, 94, 169Chrome alum, use of, 28, 32Clark, G. L., 128Clark, W., lI7Clcrc, L. P., 5, 19, 140, 238, 247Cine base, tinted, 190

negative emulsions, 187positive, 188

Coating machines, film, 176, 181,182, 184

glass, 147paper, 162spreader, 148

Coating weizht analyses, 231Color contrast, 106

formers, i:2$sensitiveness, 226sensitizers, 104

Contrast in emulsions, 99Copper printing paper, 249

salts, 249Covering power, 166Crocks, emulsion, 43Cupric chloride, use of, 97

D

Daguerre, Louis Jacqucs Mande, IDarkroom equipment, 42

260 PHOTOGRAPHIC EMULSIOK TECKJIQUEillumination, 40layout, 38

Davidson, L. F., 97Defender tripack, II9Densitometers, 210, 217, 220, 223Density range of emulsions, 19, 204Density, diffuse, 220

parallel, 220Dewar flasks, 47, 216Diazotype printing, 249Dicyanin, II4, II6Dieterle and Reister, II6Diffraction grating device, 227Digesting pans, 47, 57Digestion of emulsions, 47, 73

time of, 75vs speed, 74

Dimitroff, G. Z., 215Dixon coating machinery, 162Duclaux and J anet, 135Dufaycolor film, 81, 82

E

Eder, J. M., x, 13, 66, 79, 107, 108,212

Eggert and Noddack, 231Ellis and Well , 135Emulsions, bleach out, 2$1

boiled,6bromide paper, 166, 168, 169chloride, 3, 88, 94, 97, 98, 99,170chloro-bromide, 94, 169cine negative, 68, 187cine positive, 63, 188collodion, 30, 199color-sensitivc, J04

comparative sensitivity, 103document, 79film pack, II 7formulas, types of, 64infrared, 1I5keeping tests, 233lantern, 63, 89, 93Lippmann, 135mixed jet, 100, 188orthochromatic, 79, 105panchromatic, 105photogravure, 91positi ve, r88

process, 63, 89, 91reversal, 81, 83Schumann, 134self-filtering, 123shredding, 55silver phosphate, ~oslow, Chapter Vtransparency, 3, 63, 93, 99, 102ultraviolet, 134X-ray, 132

Engler viscometer, 24Eosin, 105, IT2Erythrosin, 105, 107, 109, 1I2

F

Fabrics, sensitizing, 195, 242Fcertype, 2SoFerguson, W. B., 220Ferric compounds, sensitivity of,

242Ferroprussiate paper, 241Film base, acetate, 175

nitrate, 175triacetate, 175

Film, double coated, 121substratum, 178substratuming machines, 178triple coated, 125Xvray, l.ll

Filtering, 50, 60Filters, light, 1I8, 121Finishing of emulsions, 73Fluorescent screens, 130, 133Fog, 173, 208Fog tests, 90, 234Fog-retarding compounds, 79Formula, structure of a, 64Friedman, J. S., 12

G

Gamma and grain size, ISand wavelength, 207infinity, 208time curves, 206

Gaspar, Bela, 122Gelatin, characteristics of, 21,30

choice of, 22, 187sensitizing action of, 5

Gelatino-chloride emulsions, 2Glass substratum, 139

washing of, 138Goldberg, 221Grain aggregates, 94

photo-micrography of, 10, 18, 45Grain size and speed, 7, IS, 63, 83,

86, 88Grain.size vs gradation, 77Grain size vs time of precipitation,

7Grain structure, 18Grinten, F. van del', 251Gum bichromate, 240

H

Halation, prevention of, 79, 93, 179Halides, combining proportions of,

65Hardening of emulsions, 28Herzberg, W., 156Heyne, W., 68, 187Hubbard and Carrell, 13, 16, 17, 72Hurter and Driffield, 202, 208, 212Huzii, M., 21Hypersensitive emulsions, ~S4Hypersensitizing by bathing, 83,

Ill, 253

I

Ice water, 54Illumination of darkrooms, 40Imbibition films, 190Inertia, 205

regression of, 209Infrared sensitive emulsions, lISIntensifying screens, 130Iodides in emulsions, 12, 1I2Iron printing processes, 241Isocyanins, 1I 2 .

JJahr, H., 77, 91, 96, 154J ones, Chapman, 97J ones, L. A., 22$

I JDEX

K

Kallitype, 244Keeping of emulsion, 17, So, 234

tests, 233Kendall, J. P., 256Koebig, August, 52Koegel and Steigmann, 2$0Kopke, 24, 36Kryptocyanins, 1I6Kuhn, H., 121

L

Laboratory layout, 39, 53Lantern emulsions, 89Lea, M. Carey, 17Lehmann, 136Levelling tables, 143Levy and West, 133Levy, L. A., 132Lippmann emulsions, 135Lumiere, A. and L., 81Lumiere and Seyewetz, 236Luther, R., 257Lyman, T., 135

M

Maddox, R. L., ix, 2, 23Mannes and Godowsky, 122Materials for emulsion making, 33Mees, C. E. K., Ill, 222Melting point determinations, 25Microscopy of grains, 10, 18, 45Mixed jet emulsions, 100Mixing of emulsions, 66, 77, 94Monopack emulsions, 121, 123Morgan and Lester, 20$

N

Naphthol yellow, 106, lI8, 120orange, 93eocyanin, 1I6

[essler's reagent, use of, 72Niepce de St. Victor, 22

ietz, A. H., 208itrate film base, I75on-stress coating, 159

Noodle press, 54, 55

Safelights, 40Salted paper, 192Scheelite, 130Schinzel, K., 122, 124, 125Schumann emulsions, 134Schwarzchild factor, 19Sensitivity, cause of, 5, 253

relative, 5Setting point, determination of, 27Shepherd, E. Sanger, 189, 220Sheppard, S. E., x, 2, 3, 5, 6, 10,

23, 27, 256Sheppard and Houck, 2Sheppard and Mees, 212Shredder, hydraulic, 55Silver phosphate sensitizer, 200Smith, J. H., 6, 252Sodium sulphite sensitizer, 16South worth, 7Spectral response, 103, 104, 106Spectrograph, construction of, 226Spreaders, 148Stabilizers, 256Staud and Connelly, 2Stelling, C., 24Step-wedges, 210, 230Sticky-back lass, 184Stirring apparatus, 101Storr, B. V., 23Strong, J., 19Stripping collodion, 30, 200Structure of a formula, 64Substratum for film base, 178

for glass plates, 140Substratuming machines, 178Super-sensitization, IIISvedberg, The, 17

o

PHOTOGRAPHIC EMULSION TECH fIQUE

S

Oakley, C. F., 80Opacity, 203Orthochromatic emulsions, 105Orthochrome, T., lIO, lII, 114Ostwald ripening, 10, 66

viscometer, 24Ozafilm, 250Ozalid process, 250

P

Packing material, 153Palladiotype paper, 249Pans, digesting, 47Paper base, 156Pepper, 94, 174pH determinations, 232pH of emulsions, 72Pho phate emulsions, 200Photogravure emulsions, 88, 91Photometers, 212Photomicrography of grains, 18, 45Pinacyanol, Ill, 123Pinaflavol, 109, Ill, Il2Pinaverdol, lIO, IIIPizzighelli, J., 244Plate coating, 144Plate coating machines, 147, ISOPlate testing, 2 ISPlatinotype paper, 246Ponton, Mungo, 23Printing-out papers, 196Proce emulsions, 89Punnett, R. F., 5

R

Raw paper, 156Reciprocity law, 19Reeling machinery, 180Renwick, F. F., 107,215,225Resolution, 20, 87Resolving power, 20Reversal emulsions, 81, 84Rhodamin, 112Ripening, 10, 13

T

Tallent, A. K., 226Tartrazin, 106, 120Tetrocarbocyanin, II 6Testing equipment, 210Thallium chloride, 171Thermometers, types of, 42Thermo-regulators, 58

Thermos flasks, 58Thermostatic control, 57Thiebaut, A., 249Thiosinamin, 256Thiourea, 256Three layer coatings, lI8, 122, 125,

191Tien Kin, 134Tissue, carbon, 238Toy and Rawling, 223Triacetate base, 175Tripacks, 117Trist, Ronald, 236Trivelli, 3, 6Trivelli and Shepard, 3Trivelli and Smith, 6, 13, 14, 19, 74,

258Truum, A., 199Turbidity factor, 221

U

Ultraviolet absorption by gelatin,134

Ultraviolet emulsions, 134Unwashed emulsions, 169Uranium process, 249

V

Valenta, E., 95, 96, 200V-spreader, 148Viscosimeters, 24Viscosity of gelatin, 24Visual spectral response, 103, 104,

106Vogel, H. W., xi, 105Von Weimarn, 8

I DEX

W

Wall, E. J., 26, 66, 83, 91, 105, 168,177

Warm tones, 94, 97Washing glass, 138Washing of emulsions, 48, 56, 70Wash-Off Relief films, 189Waterhouse, 105Wavelength, scale, 229Wedge photometer, 212Wedge's for sensitometry, 210Weigert and Luhr, 64Wellington, J. B. B., 95Wiemarn, von, 8Willi , W., 246Wilsey and Pritchard, 129Wivesleigh, de, W., x

xXenocyanin, Ill)

X-opaque gelatin, 132X-ray intensifying screens, 130

emulsions, IIS, 132

Y

Yellow, brilliant, 93, 106, 118naphthol, 106, II8thiazol, 106

·Z

Zinc sulphide, 133X-ray screens, 133

1"1' •

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Photographic Emulsion Technique